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  97.     <ul>

  98. <li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code> — Process-based parallelism</a><ul>

  99. <li><a class="reference internal" href="#introduction">Introduction</a><ul>

  100. <li><a class="reference internal" href="#the-process-class">The <code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code> class</a></li>

  101. <li><a class="reference internal" href="#contexts-and-start-methods">Contexts and start methods</a></li>

  102. <li><a class="reference internal" href="#exchanging-objects-between-processes">Exchanging objects between processes</a></li>

  103. <li><a class="reference internal" href="#synchronization-between-processes">Synchronization between processes</a></li>

  104. <li><a class="reference internal" href="#sharing-state-between-processes">Sharing state between processes</a></li>

  105. <li><a class="reference internal" href="#using-a-pool-of-workers">Using a pool of workers</a></li>

  106. </ul>

  107. </li>

  108. <li><a class="reference internal" href="#reference">Reference</a><ul>

  109. <li><a class="reference internal" href="#process-and-exceptions"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code> and exceptions</a></li>

  110. <li><a class="reference internal" href="#pipes-and-queues">Pipes and Queues</a></li>

  111. <li><a class="reference internal" href="#miscellaneous">Miscellaneous</a></li>

  112. <li><a class="reference internal" href="#connection-objects">Connection Objects</a></li>

  113. <li><a class="reference internal" href="#synchronization-primitives">Synchronization primitives</a></li>

  114. <li><a class="reference internal" href="#shared-ctypes-objects">Shared <code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code> Objects</a><ul>

  115. <li><a class="reference internal" href="#module-multiprocessing.sharedctypes">The <code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.sharedctypes</span></code> module</a></li>

  116. </ul>

  117. </li>

  118. <li><a class="reference internal" href="#managers">Managers</a><ul>

  119. <li><a class="reference internal" href="#customized-managers">Customized managers</a></li>

  120. <li><a class="reference internal" href="#using-a-remote-manager">Using a remote manager</a></li>

  121. </ul>

  122. </li>

  123. <li><a class="reference internal" href="#proxy-objects">Proxy Objects</a><ul>

  124. <li><a class="reference internal" href="#cleanup">Cleanup</a></li>

  125. </ul>

  126. </li>

  127. <li><a class="reference internal" href="#module-multiprocessing.pool">Process Pools</a></li>

  128. <li><a class="reference internal" href="#module-multiprocessing.connection">Listeners and Clients</a><ul>

  129. <li><a class="reference internal" href="#address-formats">Address Formats</a></li>

  130. </ul>

  131. </li>

  132. <li><a class="reference internal" href="#authentication-keys">Authentication keys</a></li>

  133. <li><a class="reference internal" href="#logging">Logging</a></li>

  134. <li><a class="reference internal" href="#module-multiprocessing.dummy">The <code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.dummy</span></code> module</a></li>

  135. </ul>

  136. </li>

  137. <li><a class="reference internal" href="#programming-guidelines">Programming guidelines</a><ul>

  138. <li><a class="reference internal" href="#all-start-methods">All start methods</a></li>

  139. <li><a class="reference internal" href="#the-spawn-and-forkserver-start-methods">The <em>spawn</em> and <em>forkserver</em> start methods</a></li>

  140. </ul>

  141. </li>

  142. <li><a class="reference internal" href="#examples">Examples</a></li>

  143. </ul>

  144. </li>

  145. </ul>

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  151.                           title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code> — Thread-based parallelism</a></p>

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  156.                           title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.shared_memory</span></code> — Shared memory for direct access across processes</a></p>

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  235.   <section id="module-multiprocessing">

  236. <span id="multiprocessing-process-based-parallelism"></span><h1><a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> — Process-based parallelism<a class="headerlink" href="#module-multiprocessing" title="Link to this heading">¶</a></h1>

  237. <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.12/Lib/multiprocessing/">Lib/multiprocessing/</a></p>

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  240. <p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: not Emscripten, not WASI.</p>

  241. <p>This module does not work or is not available on WebAssembly platforms

  242. <code class="docutils literal notranslate"><span class="pre">wasm32-emscripten</span></code> and <code class="docutils literal notranslate"><span class="pre">wasm32-wasi</span></code>. See

  243. <a class="reference internal" href="intro.html#wasm-availability"><span class="std std-ref">WebAssembly platforms</span></a> for more information.</p>

  244. </div>

  245. <section id="introduction">

  246. <h2>Introduction<a class="headerlink" href="#introduction" title="Link to this heading">¶</a></h2>

  247. <p><a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> is a package that supports spawning processes using an

  248. API similar to the <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module.  The <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> package

  249. offers both local and remote concurrency, effectively side-stepping the

  250. <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><span class="xref std std-term">Global Interpreter Lock</span></a> by using

  251. subprocesses instead of threads.  Due

  252. to this, the <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> module allows the programmer to fully

  253. leverage multiple processors on a given machine.  It runs on both POSIX and

  254. Windows.</p>

  255. <p>The <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> module also introduces APIs which do not have

  256. analogs in the <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module.  A prime example of this is the

  257. <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a> object which offers a convenient means of

  258. parallelizing the execution of a function across multiple input values,

  259. distributing the input data across processes (data parallelism).  The following

  260. example demonstrates the common practice of defining such functions in a module

  261. so that child processes can successfully import that module.  This basic example

  262. of data parallelism using <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a>,</p>

  263. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Pool</span>

  264. 

  265. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>

  266.     <span class="k">return</span> <span class="n">x</span><span class="o">*</span><span class="n">x</span>

  267. 

  268. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  269.     <span class="k">with</span> <span class="n">Pool</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="k">as</span> <span class="n">p</span><span class="p">:</span>

  270.         <span class="nb">print</span><span class="p">(</span><span class="n">p</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]))</span>

  271. </pre></div>

  272. </div>

  273. <p>will print to standard output</p>

  274. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">9</span><span class="p">]</span>

  275. </pre></div>

  276. </div>

  277. <div class="admonition seealso">

  278. <p class="admonition-title">See also</p>

  279. <p><a class="reference internal" href="concurrent.futures.html#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures.ProcessPoolExecutor</span></code></a> offers a higher level interface

  280. to push tasks to a background process without blocking execution of the

  281. calling process. Compared to using the <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a>

  282. interface directly, the <a class="reference internal" href="concurrent.futures.html#module-concurrent.futures" title="concurrent.futures: Execute computations concurrently using threads or processes."><code class="xref py py-mod docutils literal notranslate"><span class="pre">concurrent.futures</span></code></a> API more readily allows

  283. the submission of work to the underlying process pool to be separated from

  284. waiting for the results.</p>

  285. </div>

  286. <section id="the-process-class">

  287. <h3>The <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> class<a class="headerlink" href="#the-process-class" title="Link to this heading">¶</a></h3>

  288. <p>In <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a>, processes are spawned by creating a <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a>

  289. object and then calling its <a class="reference internal" href="#multiprocessing.Process.start" title="multiprocessing.Process.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">start()</span></code></a> method.  <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a>

  290. follows the API of <a class="reference internal" href="threading.html#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Thread</span></code></a>.  A trivial example of a

  291. multiprocess program is</p>

  292. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span>

  293. 

  294. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">name</span><span class="p">):</span>

  295.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;hello&#39;</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span>

  296. 

  297. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  298.     <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;bob&#39;</span><span class="p">,))</span>

  299.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  300.     <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  301. </pre></div>

  302. </div>

  303. <p>To show the individual process IDs involved, here is an expanded example:</p>

  304. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span>

  305. <span class="kn">import</span> <span class="nn">os</span>

  306. 

  307. <span class="k">def</span> <span class="nf">info</span><span class="p">(</span><span class="n">title</span><span class="p">):</span>

  308.     <span class="nb">print</span><span class="p">(</span><span class="n">title</span><span class="p">)</span>

  309.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;module name:&#39;</span><span class="p">,</span> <span class="vm">__name__</span><span class="p">)</span>

  310.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;parent process:&#39;</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">getppid</span><span class="p">())</span>

  311.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;process id:&#39;</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">getpid</span><span class="p">())</span>

  312. 

  313. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">name</span><span class="p">):</span>

  314.     <span class="n">info</span><span class="p">(</span><span class="s1">&#39;function f&#39;</span><span class="p">)</span>

  315.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;hello&#39;</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span>

  316. 

  317. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  318.     <span class="n">info</span><span class="p">(</span><span class="s1">&#39;main line&#39;</span><span class="p">)</span>

  319.     <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;bob&#39;</span><span class="p">,))</span>

  320.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  321.     <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  322. </pre></div>

  323. </div>

  324. <p>For an explanation of why the <code class="docutils literal notranslate"><span class="pre">if</span> <span class="pre">__name__</span> <span class="pre">==</span> <span class="pre">'__main__'</span></code> part is

  325. necessary, see <a class="reference internal" href="#multiprocessing-programming"><span class="std std-ref">Programming guidelines</span></a>.</p>

  326. </section>

  327. <section id="contexts-and-start-methods">

  328. <span id="multiprocessing-start-methods"></span><h3>Contexts and start methods<a class="headerlink" href="#contexts-and-start-methods" title="Link to this heading">¶</a></h3>

  329. <p>Depending on the platform, <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> supports three ways

  330. to start a process.  These <em>start methods</em> are</p>

  331. <blockquote>

  332. <div><dl>

  333. <dt><em>spawn</em></dt><dd><p>The parent process starts a fresh Python interpreter process.  The

  334. child process will only inherit those resources necessary to run

  335. the process object’s <a class="reference internal" href="#multiprocessing.Process.run" title="multiprocessing.Process.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method.  In particular,

  336. unnecessary file descriptors and handles from the parent process

  337. will not be inherited.  Starting a process using this method is

  338. rather slow compared to using <em>fork</em> or <em>forkserver</em>.</p>

  339. <p>Available on POSIX and Windows platforms.  The default on Windows and macOS.</p>

  340. </dd>

  341. <dt><em>fork</em></dt><dd><p>The parent process uses <a class="reference internal" href="os.html#os.fork" title="os.fork"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.fork()</span></code></a> to fork the Python

  342. interpreter.  The child process, when it begins, is effectively

  343. identical to the parent process.  All resources of the parent are

  344. inherited by the child process.  Note that safely forking a

  345. multithreaded process is problematic.</p>

  346. <p>Available on POSIX systems.  Currently the default on POSIX except macOS.</p>

  347. <div class="admonition note">

  348. <p class="admonition-title">Note</p>

  349. <p>The default start method will change away from <em>fork</em> in Python 3.14.

  350. Code that requires <em>fork</em> should explicitly specify that via

  351. <a class="reference internal" href="#multiprocessing.get_context" title="multiprocessing.get_context"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_context()</span></code></a> or <a class="reference internal" href="#multiprocessing.set_start_method" title="multiprocessing.set_start_method"><code class="xref py py-func docutils literal notranslate"><span class="pre">set_start_method()</span></code></a>.</p>

  352. </div>

  353. <div class="versionchanged">

  354. <p><span class="versionmodified changed">Changed in version 3.12: </span>If Python is able to detect that your process has multiple threads, the

  355. <a class="reference internal" href="os.html#os.fork" title="os.fork"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.fork()</span></code></a> function that this start method calls internally will

  356. raise a <a class="reference internal" href="exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">DeprecationWarning</span></code></a>. Use a different start method.

  357. See the <a class="reference internal" href="os.html#os.fork" title="os.fork"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.fork()</span></code></a> documentation for further explanation.</p>

  358. </div>

  359. </dd>

  360. <dt><em>forkserver</em></dt><dd><p>When the program starts and selects the <em>forkserver</em> start method,

  361. a server process is spawned.  From then on, whenever a new process

  362. is needed, the parent process connects to the server and requests

  363. that it fork a new process.  The fork server process is single threaded

  364. unless system libraries or preloaded imports spawn threads as a

  365. side-effect so it is generally safe for it to use <a class="reference internal" href="os.html#os.fork" title="os.fork"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.fork()</span></code></a>.

  366. No unnecessary resources are inherited.</p>

  367. <p>Available on POSIX platforms which support passing file descriptors

  368. over Unix pipes such as Linux.</p>

  369. </dd>

  370. </dl>

  371. </div></blockquote>

  372. <div class="versionchanged">

  373. <p><span class="versionmodified changed">Changed in version 3.4: </span><em>spawn</em> added on all POSIX platforms, and <em>forkserver</em> added for

  374. some POSIX platforms.

  375. Child processes no longer inherit all of the parents inheritable

  376. handles on Windows.</p>

  377. </div>

  378. <div class="versionchanged">

  379. <p><span class="versionmodified changed">Changed in version 3.8: </span>On macOS, the <em>spawn</em> start method is now the default.  The <em>fork</em> start

  380. method should be considered unsafe as it can lead to crashes of the

  381. subprocess as macOS system libraries may start threads. See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33725">bpo-33725</a>.</p>

  382. </div>

  383. <p>On POSIX using the <em>spawn</em> or <em>forkserver</em> start methods will also

  384. start a <em>resource tracker</em> process which tracks the unlinked named

  385. system resources (such as named semaphores or

  386. <a class="reference internal" href="multiprocessing.shared_memory.html#multiprocessing.shared_memory.SharedMemory" title="multiprocessing.shared_memory.SharedMemory"><code class="xref py py-class docutils literal notranslate"><span class="pre">SharedMemory</span></code></a> objects) created

  387. by processes of the program.  When all processes

  388. have exited the resource tracker unlinks any remaining tracked object.

  389. Usually there should be none, but if a process was killed by a signal

  390. there may be some “leaked” resources.  (Neither leaked semaphores nor shared

  391. memory segments will be automatically unlinked until the next reboot. This is

  392. problematic for both objects because the system allows only a limited number of

  393. named semaphores, and shared memory segments occupy some space in the main

  394. memory.)</p>

  395. <p>To select a start method you use the <a class="reference internal" href="#multiprocessing.set_start_method" title="multiprocessing.set_start_method"><code class="xref py py-func docutils literal notranslate"><span class="pre">set_start_method()</span></code></a> in

  396. the <code class="docutils literal notranslate"><span class="pre">if</span> <span class="pre">__name__</span> <span class="pre">==</span> <span class="pre">'__main__'</span></code> clause of the main module.  For

  397. example:</p>

  398. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">multiprocessing</span> <span class="k">as</span> <span class="nn">mp</span>

  399. 

  400. <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">q</span><span class="p">):</span>

  401.     <span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="s1">&#39;hello&#39;</span><span class="p">)</span>

  402. 

  403. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  404.     <span class="n">mp</span><span class="o">.</span><span class="n">set_start_method</span><span class="p">(</span><span class="s1">&#39;spawn&#39;</span><span class="p">)</span>

  405.     <span class="n">q</span> <span class="o">=</span> <span class="n">mp</span><span class="o">.</span><span class="n">Queue</span><span class="p">()</span>

  406.     <span class="n">p</span> <span class="o">=</span> <span class="n">mp</span><span class="o">.</span><span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">foo</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">q</span><span class="p">,))</span>

  407.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  408.     <span class="nb">print</span><span class="p">(</span><span class="n">q</span><span class="o">.</span><span class="n">get</span><span class="p">())</span>

  409.     <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  410. </pre></div>

  411. </div>

  412. <p><a class="reference internal" href="#multiprocessing.set_start_method" title="multiprocessing.set_start_method"><code class="xref py py-func docutils literal notranslate"><span class="pre">set_start_method()</span></code></a> should not be used more than once in the

  413. program.</p>

  414. <p>Alternatively, you can use <a class="reference internal" href="#multiprocessing.get_context" title="multiprocessing.get_context"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_context()</span></code></a> to obtain a context

  415. object.  Context objects have the same API as the multiprocessing

  416. module, and allow one to use multiple start methods in the same

  417. program.</p>

  418. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">multiprocessing</span> <span class="k">as</span> <span class="nn">mp</span>

  419. 

  420. <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">q</span><span class="p">):</span>

  421.     <span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="s1">&#39;hello&#39;</span><span class="p">)</span>

  422. 

  423. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  424.     <span class="n">ctx</span> <span class="o">=</span> <span class="n">mp</span><span class="o">.</span><span class="n">get_context</span><span class="p">(</span><span class="s1">&#39;spawn&#39;</span><span class="p">)</span>

  425.     <span class="n">q</span> <span class="o">=</span> <span class="n">ctx</span><span class="o">.</span><span class="n">Queue</span><span class="p">()</span>

  426.     <span class="n">p</span> <span class="o">=</span> <span class="n">ctx</span><span class="o">.</span><span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">foo</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">q</span><span class="p">,))</span>

  427.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  428.     <span class="nb">print</span><span class="p">(</span><span class="n">q</span><span class="o">.</span><span class="n">get</span><span class="p">())</span>

  429.     <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  430. </pre></div>

  431. </div>

  432. <p>Note that objects related to one context may not be compatible with

  433. processes for a different context.  In particular, locks created using

  434. the <em>fork</em> context cannot be passed to processes started using the

  435. <em>spawn</em> or <em>forkserver</em> start methods.</p>

  436. <p>A library which wants to use a particular start method should probably

  437. use <a class="reference internal" href="#multiprocessing.get_context" title="multiprocessing.get_context"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_context()</span></code></a> to avoid interfering with the choice of the

  438. library user.</p>

  439. <div class="admonition warning">

  440. <p class="admonition-title">Warning</p>

  441. <p>The <code class="docutils literal notranslate"><span class="pre">'spawn'</span></code> and <code class="docutils literal notranslate"><span class="pre">'forkserver'</span></code> start methods generally cannot

  442. be used with “frozen” executables (i.e., binaries produced by

  443. packages like <strong>PyInstaller</strong> and <strong>cx_Freeze</strong>) on POSIX systems.

  444. The <code class="docutils literal notranslate"><span class="pre">'fork'</span></code> start method may work if code does not use threads.</p>

  445. </div>

  446. </section>

  447. <section id="exchanging-objects-between-processes">

  448. <h3>Exchanging objects between processes<a class="headerlink" href="#exchanging-objects-between-processes" title="Link to this heading">¶</a></h3>

  449. <p><a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> supports two types of communication channel between

  450. processes:</p>

  451. <p><strong>Queues</strong></p>

  452. <blockquote>

  453. <div><p>The <a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a> class is a near clone of <a class="reference internal" href="queue.html#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">queue.Queue</span></code></a>.  For

  454. example:</p>

  455. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Queue</span>

  456. 

  457. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">q</span><span class="p">):</span>

  458.     <span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">([</span><span class="mi">42</span><span class="p">,</span> <span class="kc">None</span><span class="p">,</span> <span class="s1">&#39;hello&#39;</span><span class="p">])</span>

  459. 

  460. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  461.     <span class="n">q</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span>

  462.     <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">q</span><span class="p">,))</span>

  463.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  464.     <span class="nb">print</span><span class="p">(</span><span class="n">q</span><span class="o">.</span><span class="n">get</span><span class="p">())</span>    <span class="c1"># prints &quot;[42, None, &#39;hello&#39;]&quot;</span>

  465.     <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  466. </pre></div>

  467. </div>

  468. <p>Queues are thread and process safe.</p>

  469. </div></blockquote>

  470. <p><strong>Pipes</strong></p>

  471. <blockquote>

  472. <div><p>The <a class="reference internal" href="#multiprocessing.Pipe" title="multiprocessing.Pipe"><code class="xref py py-func docutils literal notranslate"><span class="pre">Pipe()</span></code></a> function returns a pair of connection objects connected by a

  473. pipe which by default is duplex (two-way).  For example:</p>

  474. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Pipe</span>

  475. 

  476. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">conn</span><span class="p">):</span>

  477.     <span class="n">conn</span><span class="o">.</span><span class="n">send</span><span class="p">([</span><span class="mi">42</span><span class="p">,</span> <span class="kc">None</span><span class="p">,</span> <span class="s1">&#39;hello&#39;</span><span class="p">])</span>

  478.     <span class="n">conn</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>

  479. 

  480. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  481.     <span class="n">parent_conn</span><span class="p">,</span> <span class="n">child_conn</span> <span class="o">=</span> <span class="n">Pipe</span><span class="p">()</span>

  482.     <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">child_conn</span><span class="p">,))</span>

  483.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  484.     <span class="nb">print</span><span class="p">(</span><span class="n">parent_conn</span><span class="o">.</span><span class="n">recv</span><span class="p">())</span>   <span class="c1"># prints &quot;[42, None, &#39;hello&#39;]&quot;</span>

  485.     <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  486. </pre></div>

  487. </div>

  488. <p>The two connection objects returned by <a class="reference internal" href="#multiprocessing.Pipe" title="multiprocessing.Pipe"><code class="xref py py-func docutils literal notranslate"><span class="pre">Pipe()</span></code></a> represent the two ends of

  489. the pipe.  Each connection object has <code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code> and

  490. <code class="xref py py-meth docutils literal notranslate"><span class="pre">recv()</span></code> methods (among others).  Note that data in a pipe

  491. may become corrupted if two processes (or threads) try to read from or write

  492. to the <em>same</em> end of the pipe at the same time.  Of course there is no risk

  493. of corruption from processes using different ends of the pipe at the same

  494. time.</p>

  495. </div></blockquote>

  496. </section>

  497. <section id="synchronization-between-processes">

  498. <h3>Synchronization between processes<a class="headerlink" href="#synchronization-between-processes" title="Link to this heading">¶</a></h3>

  499. <p><a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> contains equivalents of all the synchronization

  500. primitives from <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a>.  For instance one can use a lock to ensure

  501. that only one process prints to standard output at a time:</p>

  502. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Lock</span>

  503. 

  504. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">l</span><span class="p">,</span> <span class="n">i</span><span class="p">):</span>

  505.     <span class="n">l</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>

  506.     <span class="k">try</span><span class="p">:</span>

  507.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;hello world&#39;</span><span class="p">,</span> <span class="n">i</span><span class="p">)</span>

  508.     <span class="k">finally</span><span class="p">:</span>

  509.         <span class="n">l</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>

  510. 

  511. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  512.     <span class="n">lock</span> <span class="o">=</span> <span class="n">Lock</span><span class="p">()</span>

  513. 

  514.     <span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>

  515.         <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">lock</span><span class="p">,</span> <span class="n">num</span><span class="p">))</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  516. </pre></div>

  517. </div>

  518. <p>Without using the lock output from the different processes is liable to get all

  519. mixed up.</p>

  520. </section>

  521. <section id="sharing-state-between-processes">

  522. <h3>Sharing state between processes<a class="headerlink" href="#sharing-state-between-processes" title="Link to this heading">¶</a></h3>

  523. <p>As mentioned above, when doing concurrent programming it is usually best to

  524. avoid using shared state as far as possible.  This is particularly true when

  525. using multiple processes.</p>

  526. <p>However, if you really do need to use some shared data then

  527. <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> provides a couple of ways of doing so.</p>

  528. <p><strong>Shared memory</strong></p>

  529. <blockquote>

  530. <div><p>Data can be stored in a shared memory map using <a class="reference internal" href="#multiprocessing.Value" title="multiprocessing.Value"><code class="xref py py-class docutils literal notranslate"><span class="pre">Value</span></code></a> or

  531. <a class="reference internal" href="#multiprocessing.Array" title="multiprocessing.Array"><code class="xref py py-class docutils literal notranslate"><span class="pre">Array</span></code></a>.  For example, the following code</p>

  532. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Value</span><span class="p">,</span> <span class="n">Array</span>

  533. 

  534. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="n">a</span><span class="p">):</span>

  535.     <span class="n">n</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="mf">3.1415927</span>

  536.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">a</span><span class="p">)):</span>

  537.         <span class="n">a</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="o">-</span><span class="n">a</span><span class="p">[</span><span class="n">i</span><span class="p">]</span>

  538. 

  539. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  540.     <span class="n">num</span> <span class="o">=</span> <span class="n">Value</span><span class="p">(</span><span class="s1">&#39;d&#39;</span><span class="p">,</span> <span class="mf">0.0</span><span class="p">)</span>

  541.     <span class="n">arr</span> <span class="o">=</span> <span class="n">Array</span><span class="p">(</span><span class="s1">&#39;i&#39;</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>

  542. 

  543.     <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">num</span><span class="p">,</span> <span class="n">arr</span><span class="p">))</span>

  544.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  545.     <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  546. 

  547.     <span class="nb">print</span><span class="p">(</span><span class="n">num</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>

  548.     <span class="nb">print</span><span class="p">(</span><span class="n">arr</span><span class="p">[:])</span>

  549. </pre></div>

  550. </div>

  551. <p>will print</p>

  552. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="mf">3.1415927</span>

  553. <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="o">-</span><span class="mi">2</span><span class="p">,</span> <span class="o">-</span><span class="mi">3</span><span class="p">,</span> <span class="o">-</span><span class="mi">4</span><span class="p">,</span> <span class="o">-</span><span class="mi">5</span><span class="p">,</span> <span class="o">-</span><span class="mi">6</span><span class="p">,</span> <span class="o">-</span><span class="mi">7</span><span class="p">,</span> <span class="o">-</span><span class="mi">8</span><span class="p">,</span> <span class="o">-</span><span class="mi">9</span><span class="p">]</span>

  554. </pre></div>

  555. </div>

  556. <p>The <code class="docutils literal notranslate"><span class="pre">'d'</span></code> and <code class="docutils literal notranslate"><span class="pre">'i'</span></code> arguments used when creating <code class="docutils literal notranslate"><span class="pre">num</span></code> and <code class="docutils literal notranslate"><span class="pre">arr</span></code> are

  557. typecodes of the kind used by the <a class="reference internal" href="array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><code class="xref py py-mod docutils literal notranslate"><span class="pre">array</span></code></a> module: <code class="docutils literal notranslate"><span class="pre">'d'</span></code> indicates a

  558. double precision float and <code class="docutils literal notranslate"><span class="pre">'i'</span></code> indicates a signed integer.  These shared

  559. objects will be process and thread-safe.</p>

  560. <p>For more flexibility in using shared memory one can use the

  561. <a class="reference internal" href="#module-multiprocessing.sharedctypes" title="multiprocessing.sharedctypes: Allocate ctypes objects from shared memory."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.sharedctypes</span></code></a> module which supports the creation of

  562. arbitrary ctypes objects allocated from shared memory.</p>

  563. </div></blockquote>

  564. <p><strong>Server process</strong></p>

  565. <blockquote>

  566. <div><p>A manager object returned by <a class="reference internal" href="#multiprocessing.Manager" title="multiprocessing.Manager"><code class="xref py py-func docutils literal notranslate"><span class="pre">Manager()</span></code></a> controls a server process which

  567. holds Python objects and allows other processes to manipulate them using

  568. proxies.</p>

  569. <p>A manager returned by <a class="reference internal" href="#multiprocessing.Manager" title="multiprocessing.Manager"><code class="xref py py-func docutils literal notranslate"><span class="pre">Manager()</span></code></a> will support types

  570. <a class="reference internal" href="stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate"><span class="pre">list</span></code></a>, <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a>, <a class="reference internal" href="#multiprocessing.managers.Namespace" title="multiprocessing.managers.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a>, <a class="reference internal" href="#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a>,

  571. <a class="reference internal" href="#multiprocessing.RLock" title="multiprocessing.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a>, <a class="reference internal" href="#multiprocessing.Semaphore" title="multiprocessing.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">Semaphore</span></code></a>, <a class="reference internal" href="#multiprocessing.BoundedSemaphore" title="multiprocessing.BoundedSemaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">BoundedSemaphore</span></code></a>,

  572. <a class="reference internal" href="#multiprocessing.Condition" title="multiprocessing.Condition"><code class="xref py py-class docutils literal notranslate"><span class="pre">Condition</span></code></a>, <a class="reference internal" href="#multiprocessing.Event" title="multiprocessing.Event"><code class="xref py py-class docutils literal notranslate"><span class="pre">Event</span></code></a>, <a class="reference internal" href="#multiprocessing.Barrier" title="multiprocessing.Barrier"><code class="xref py py-class docutils literal notranslate"><span class="pre">Barrier</span></code></a>,

  573. <a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a>, <a class="reference internal" href="#multiprocessing.Value" title="multiprocessing.Value"><code class="xref py py-class docutils literal notranslate"><span class="pre">Value</span></code></a> and <a class="reference internal" href="#multiprocessing.Array" title="multiprocessing.Array"><code class="xref py py-class docutils literal notranslate"><span class="pre">Array</span></code></a>.  For example,</p>

  574. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Manager</span>

  575. 

  576. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">d</span><span class="p">,</span> <span class="n">l</span><span class="p">):</span>

  577.     <span class="n">d</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="s1">&#39;1&#39;</span>

  578.     <span class="n">d</span><span class="p">[</span><span class="s1">&#39;2&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="mi">2</span>

  579.     <span class="n">d</span><span class="p">[</span><span class="mf">0.25</span><span class="p">]</span> <span class="o">=</span> <span class="kc">None</span>

  580.     <span class="n">l</span><span class="o">.</span><span class="n">reverse</span><span class="p">()</span>

  581. 

  582. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  583.     <span class="k">with</span> <span class="n">Manager</span><span class="p">()</span> <span class="k">as</span> <span class="n">manager</span><span class="p">:</span>

  584.         <span class="n">d</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">dict</span><span class="p">()</span>

  585.         <span class="n">l</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>

  586. 

  587.         <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">d</span><span class="p">,</span> <span class="n">l</span><span class="p">))</span>

  588.         <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  589.         <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  590. 

  591.         <span class="nb">print</span><span class="p">(</span><span class="n">d</span><span class="p">)</span>

  592.         <span class="nb">print</span><span class="p">(</span><span class="n">l</span><span class="p">)</span>

  593. </pre></div>

  594. </div>

  595. <p>will print</p>

  596. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="p">{</span><span class="mf">0.25</span><span class="p">:</span> <span class="kc">None</span><span class="p">,</span> <span class="mi">1</span><span class="p">:</span> <span class="s1">&#39;1&#39;</span><span class="p">,</span> <span class="s1">&#39;2&#39;</span><span class="p">:</span> <span class="mi">2</span><span class="p">}</span>

  597. <span class="p">[</span><span class="mi">9</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">]</span>

  598. </pre></div>

  599. </div>

  600. <p>Server process managers are more flexible than using shared memory objects

  601. because they can be made to support arbitrary object types.  Also, a single

  602. manager can be shared by processes on different computers over a network.

  603. They are, however, slower than using shared memory.</p>

  604. </div></blockquote>

  605. </section>

  606. <section id="using-a-pool-of-workers">

  607. <h3>Using a pool of workers<a class="headerlink" href="#using-a-pool-of-workers" title="Link to this heading">¶</a></h3>

  608. <p>The <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a> class represents a pool of worker

  609. processes.  It has methods which allows tasks to be offloaded to the worker

  610. processes in a few different ways.</p>

  611. <p>For example:</p>

  612. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Pool</span><span class="p">,</span> <span class="ne">TimeoutError</span>

  613. <span class="kn">import</span> <span class="nn">time</span>

  614. <span class="kn">import</span> <span class="nn">os</span>

  615. 

  616. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>

  617.     <span class="k">return</span> <span class="n">x</span><span class="o">*</span><span class="n">x</span>

  618. 

  619. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  620.     <span class="c1"># start 4 worker processes</span>

  621.     <span class="k">with</span> <span class="n">Pool</span><span class="p">(</span><span class="n">processes</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span> <span class="k">as</span> <span class="n">pool</span><span class="p">:</span>

  622. 

  623.         <span class="c1"># print &quot;[0, 1, 4,..., 81]&quot;</span>

  624.         <span class="nb">print</span><span class="p">(</span><span class="n">pool</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)))</span>

  625. 

  626.         <span class="c1"># print same numbers in arbitrary order</span>

  627.         <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">pool</span><span class="o">.</span><span class="n">imap_unordered</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)):</span>

  628.             <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>

  629. 

  630.         <span class="c1"># evaluate &quot;f(20)&quot; asynchronously</span>

  631.         <span class="n">res</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">apply_async</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="p">(</span><span class="mi">20</span><span class="p">,))</span>      <span class="c1"># runs in *only* one process</span>

  632.         <span class="nb">print</span><span class="p">(</span><span class="n">res</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">1</span><span class="p">))</span>             <span class="c1"># prints &quot;400&quot;</span>

  633. 

  634.         <span class="c1"># evaluate &quot;os.getpid()&quot; asynchronously</span>

  635.         <span class="n">res</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">apply_async</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">getpid</span><span class="p">,</span> <span class="p">())</span> <span class="c1"># runs in *only* one process</span>

  636.         <span class="nb">print</span><span class="p">(</span><span class="n">res</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">1</span><span class="p">))</span>             <span class="c1"># prints the PID of that process</span>

  637. 

  638.         <span class="c1"># launching multiple evaluations asynchronously *may* use more processes</span>

  639.         <span class="n">multiple_results</span> <span class="o">=</span> <span class="p">[</span><span class="n">pool</span><span class="o">.</span><span class="n">apply_async</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">getpid</span><span class="p">,</span> <span class="p">())</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)]</span>

  640.         <span class="nb">print</span><span class="p">([</span><span class="n">res</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span> <span class="k">for</span> <span class="n">res</span> <span class="ow">in</span> <span class="n">multiple_results</span><span class="p">])</span>

  641. 

  642.         <span class="c1"># make a single worker sleep for 10 seconds</span>

  643.         <span class="n">res</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">apply_async</span><span class="p">(</span><span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">,</span> <span class="p">(</span><span class="mi">10</span><span class="p">,))</span>

  644.         <span class="k">try</span><span class="p">:</span>

  645.             <span class="nb">print</span><span class="p">(</span><span class="n">res</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">1</span><span class="p">))</span>

  646.         <span class="k">except</span> <span class="ne">TimeoutError</span><span class="p">:</span>

  647.             <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;We lacked patience and got a multiprocessing.TimeoutError&quot;</span><span class="p">)</span>

  648. 

  649.         <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;For the moment, the pool remains available for more work&quot;</span><span class="p">)</span>

  650. 

  651.     <span class="c1"># exiting the &#39;with&#39;-block has stopped the pool</span>

  652.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Now the pool is closed and no longer available&quot;</span><span class="p">)</span>

  653. </pre></div>

  654. </div>

  655. <p>Note that the methods of a pool should only ever be used by the

  656. process which created it.</p>

  657. <div class="admonition note">

  658. <p class="admonition-title">Note</p>

  659. <p>Functionality within this package requires that the <code class="docutils literal notranslate"><span class="pre">__main__</span></code> module be

  660. importable by the children. This is covered in <a class="reference internal" href="#multiprocessing-programming"><span class="std std-ref">Programming guidelines</span></a>

  661. however it is worth pointing out here. This means that some examples, such

  662. as the <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">multiprocessing.pool.Pool</span></code></a> examples will not work in the

  663. interactive interpreter. For example:</p>

  664. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Pool</span>

  665. <span class="gp">&gt;&gt;&gt; </span><span class="n">p</span> <span class="o">=</span> <span class="n">Pool</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>

  666. <span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>

  667. <span class="gp">... </span>    <span class="k">return</span> <span class="n">x</span><span class="o">*</span><span class="n">x</span>

  668. <span class="gp">...</span>

  669. <span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">p</span><span class="p">:</span>

  670. <span class="gp">... </span>    <span class="n">p</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">])</span>

  671. <span class="go">Process PoolWorker-1:</span>

  672. <span class="go">Process PoolWorker-2:</span>

  673. <span class="go">Process PoolWorker-3:</span>

  674. <span class="gt">Traceback (most recent call last):</span>

  675. <span class="gt">Traceback (most recent call last):</span>

  676. <span class="gt">Traceback (most recent call last):</span>

  677. <span class="gr">AttributeError</span>: <span class="n">Can&#39;t get attribute &#39;f&#39; on &lt;module &#39;__main__&#39; (&lt;class &#39;_frozen_importlib.BuiltinImporter&#39;&gt;)&gt;</span>

  678. <span class="x">AttributeError: Can&#39;t get attribute &#39;f&#39; on &lt;module &#39;__main__&#39; (&lt;class &#39;_frozen_importlib.BuiltinImporter&#39;&gt;)&gt;</span>

  679. <span class="x">AttributeError: Can&#39;t get attribute &#39;f&#39; on &lt;module &#39;__main__&#39; (&lt;class &#39;_frozen_importlib.BuiltinImporter&#39;&gt;)&gt;</span>

  680. </pre></div>

  681. </div>

  682. <p>(If you try this it will actually output three full tracebacks

  683. interleaved in a semi-random fashion, and then you may have to

  684. stop the parent process somehow.)</p>

  685. </div>

  686. </section>

  687. </section>

  688. <section id="reference">

  689. <h2>Reference<a class="headerlink" href="#reference" title="Link to this heading">¶</a></h2>

  690. <p>The <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> package mostly replicates the API of the

  691. <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module.</p>

  692. <section id="process-and-exceptions">

  693. <h3><a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> and exceptions<a class="headerlink" href="#process-and-exceptions" title="Link to this heading">¶</a></h3>

  694. <dl class="py class">

  695. <dt class="sig sig-object py" id="multiprocessing.Process">

  696. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Process</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">group</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">target</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">name</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">args</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">()</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">kwargs</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">{}</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">daemon</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Process" title="Link to this definition">¶</a></dt>

  697. <dd><p>Process objects represent activity that is run in a separate process. The

  698. <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> class has equivalents of all the methods of

  699. <a class="reference internal" href="threading.html#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Thread</span></code></a>.</p>

  700. <p>The constructor should always be called with keyword arguments. <em>group</em>

  701. should always be <code class="docutils literal notranslate"><span class="pre">None</span></code>; it exists solely for compatibility with

  702. <a class="reference internal" href="threading.html#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Thread</span></code></a>.  <em>target</em> is the callable object to be invoked by

  703. the <a class="reference internal" href="#multiprocessing.Process.run" title="multiprocessing.Process.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method.  It defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>, meaning nothing is

  704. called. <em>name</em> is the process name (see <a class="reference internal" href="#multiprocessing.Process.name" title="multiprocessing.Process.name"><code class="xref py py-attr docutils literal notranslate"><span class="pre">name</span></code></a> for more details).

  705. <em>args</em> is the argument tuple for the target invocation.  <em>kwargs</em> is a

  706. dictionary of keyword arguments for the target invocation.  If provided,

  707. the keyword-only <em>daemon</em> argument sets the process <a class="reference internal" href="#multiprocessing.Process.daemon" title="multiprocessing.Process.daemon"><code class="xref py py-attr docutils literal notranslate"><span class="pre">daemon</span></code></a> flag

  708. to <code class="docutils literal notranslate"><span class="pre">True</span></code> or <code class="docutils literal notranslate"><span class="pre">False</span></code>.  If <code class="docutils literal notranslate"><span class="pre">None</span></code> (the default), this flag will be

  709. inherited from the creating process.</p>

  710. <p>By default, no arguments are passed to <em>target</em>. The <em>args</em> argument,

  711. which defaults to <code class="docutils literal notranslate"><span class="pre">()</span></code>, can be used to specify a list or tuple of the arguments

  712. to pass to <em>target</em>.</p>

  713. <p>If a subclass overrides the constructor, it must make sure it invokes the

  714. base class constructor (<code class="xref py py-meth docutils literal notranslate"><span class="pre">Process.__init__()</span></code>) before doing anything else

  715. to the process.</p>

  716. <div class="versionchanged">

  717. <p><span class="versionmodified changed">Changed in version 3.3: </span>Added the <em>daemon</em> parameter.</p>

  718. </div>

  719. <dl class="py method">

  720. <dt class="sig sig-object py" id="multiprocessing.Process.run">

  721. <span class="sig-name descname"><span class="pre">run</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Process.run" title="Link to this definition">¶</a></dt>

  722. <dd><p>Method representing the process’s activity.</p>

  723. <p>You may override this method in a subclass.  The standard <a class="reference internal" href="#multiprocessing.Process.run" title="multiprocessing.Process.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a>

  724. method invokes the callable object passed to the object’s constructor as

  725. the target argument, if any, with sequential and keyword arguments taken

  726. from the <em>args</em> and <em>kwargs</em> arguments, respectively.</p>

  727. <p>Using a list or tuple as the <em>args</em> argument passed to <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a>

  728. achieves the same effect.</p>

  729. <p>Example:</p>

  730. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span>

  731. <span class="gp">&gt;&gt;&gt; </span><span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="nb">print</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span>

  732. <span class="gp">&gt;&gt;&gt; </span><span class="n">p</span><span class="o">.</span><span class="n">run</span><span class="p">()</span>

  733. <span class="go">1</span>

  734. <span class="gp">&gt;&gt;&gt; </span><span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="nb">print</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="mi">1</span><span class="p">,))</span>

  735. <span class="gp">&gt;&gt;&gt; </span><span class="n">p</span><span class="o">.</span><span class="n">run</span><span class="p">()</span>

  736. <span class="go">1</span>

  737. </pre></div>

  738. </div>

  739. </dd></dl>

  740. 

  741. <dl class="py method">

  742. <dt class="sig sig-object py" id="multiprocessing.Process.start">

  743. <span class="sig-name descname"><span class="pre">start</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Process.start" title="Link to this definition">¶</a></dt>

  744. <dd><p>Start the process’s activity.</p>

  745. <p>This must be called at most once per process object.  It arranges for the

  746. object’s <a class="reference internal" href="#multiprocessing.Process.run" title="multiprocessing.Process.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method to be invoked in a separate process.</p>

  747. </dd></dl>

  748. 

  749. <dl class="py method">

  750. <dt class="sig sig-object py" id="multiprocessing.Process.join">

  751. <span class="sig-name descname"><span class="pre">join</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">timeout</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Process.join" title="Link to this definition">¶</a></dt>

  752. <dd><p>If the optional argument <em>timeout</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> (the default), the method

  753. blocks until the process whose <a class="reference internal" href="#multiprocessing.Process.join" title="multiprocessing.Process.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> method is called terminates.

  754. If <em>timeout</em> is a positive number, it blocks at most <em>timeout</em> seconds.

  755. Note that the method returns <code class="docutils literal notranslate"><span class="pre">None</span></code> if its process terminates or if the

  756. method times out.  Check the process’s <a class="reference internal" href="#multiprocessing.Process.exitcode" title="multiprocessing.Process.exitcode"><code class="xref py py-attr docutils literal notranslate"><span class="pre">exitcode</span></code></a> to determine if

  757. it terminated.</p>

  758. <p>A process can be joined many times.</p>

  759. <p>A process cannot join itself because this would cause a deadlock.  It is

  760. an error to attempt to join a process before it has been started.</p>

  761. </dd></dl>

  762. 

  763. <dl class="py attribute">

  764. <dt class="sig sig-object py" id="multiprocessing.Process.name">

  765. <span class="sig-name descname"><span class="pre">name</span></span><a class="headerlink" href="#multiprocessing.Process.name" title="Link to this definition">¶</a></dt>

  766. <dd><p>The process’s name.  The name is a string used for identification purposes

  767. only.  It has no semantics.  Multiple processes may be given the same

  768. name.</p>

  769. <p>The initial name is set by the constructor.  If no explicit name is

  770. provided to the constructor, a name of the form

  771. ‘Process-N<sub>1</sub>:N<sub>2</sub>:…:N<sub>k</sub>’ is constructed, where

  772. each N<sub>k</sub> is the N-th child of its parent.</p>

  773. </dd></dl>

  774. 

  775. <dl class="py method">

  776. <dt class="sig sig-object py" id="multiprocessing.Process.is_alive">

  777. <span class="sig-name descname"><span class="pre">is_alive</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Process.is_alive" title="Link to this definition">¶</a></dt>

  778. <dd><p>Return whether the process is alive.</p>

  779. <p>Roughly, a process object is alive from the moment the <a class="reference internal" href="#multiprocessing.Process.start" title="multiprocessing.Process.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">start()</span></code></a>

  780. method returns until the child process terminates.</p>

  781. </dd></dl>

  782. 

  783. <dl class="py attribute">

  784. <dt class="sig sig-object py" id="multiprocessing.Process.daemon">

  785. <span class="sig-name descname"><span class="pre">daemon</span></span><a class="headerlink" href="#multiprocessing.Process.daemon" title="Link to this definition">¶</a></dt>

  786. <dd><p>The process’s daemon flag, a Boolean value.  This must be set before

  787. <a class="reference internal" href="#multiprocessing.Process.start" title="multiprocessing.Process.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">start()</span></code></a> is called.</p>

  788. <p>The initial value is inherited from the creating process.</p>

  789. <p>When a process exits, it attempts to terminate all of its daemonic child

  790. processes.</p>

  791. <p>Note that a daemonic process is not allowed to create child processes.

  792. Otherwise a daemonic process would leave its children orphaned if it gets

  793. terminated when its parent process exits. Additionally, these are <strong>not</strong>

  794. Unix daemons or services, they are normal processes that will be

  795. terminated (and not joined) if non-daemonic processes have exited.</p>

  796. </dd></dl>

  797. 

  798. <p>In addition to the  <a class="reference internal" href="threading.html#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Thread</span></code></a> API, <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> objects

  799. also support the following attributes and methods:</p>

  800. <dl class="py attribute">

  801. <dt class="sig sig-object py" id="multiprocessing.Process.pid">

  802. <span class="sig-name descname"><span class="pre">pid</span></span><a class="headerlink" href="#multiprocessing.Process.pid" title="Link to this definition">¶</a></dt>

  803. <dd><p>Return the process ID.  Before the process is spawned, this will be

  804. <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>

  805. </dd></dl>

  806. 

  807. <dl class="py attribute">

  808. <dt class="sig sig-object py" id="multiprocessing.Process.exitcode">

  809. <span class="sig-name descname"><span class="pre">exitcode</span></span><a class="headerlink" href="#multiprocessing.Process.exitcode" title="Link to this definition">¶</a></dt>

  810. <dd><p>The child’s exit code.  This will be <code class="docutils literal notranslate"><span class="pre">None</span></code> if the process has not yet

  811. terminated.</p>

  812. <p>If the child’s <a class="reference internal" href="#multiprocessing.Process.run" title="multiprocessing.Process.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method returned normally, the exit code

  813. will be 0.  If it terminated via <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exit()</span></code></a> with an integer

  814. argument <em>N</em>, the exit code will be <em>N</em>.</p>

  815. <p>If the child terminated due to an exception not caught within

  816. <a class="reference internal" href="#multiprocessing.Process.run" title="multiprocessing.Process.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a>, the exit code will be 1.  If it was terminated by

  817. signal <em>N</em>, the exit code will be the negative value <em>-N</em>.</p>

  818. </dd></dl>

  819. 

  820. <dl class="py attribute">

  821. <dt class="sig sig-object py" id="multiprocessing.Process.authkey">

  822. <span class="sig-name descname"><span class="pre">authkey</span></span><a class="headerlink" href="#multiprocessing.Process.authkey" title="Link to this definition">¶</a></dt>

  823. <dd><p>The process’s authentication key (a byte string).</p>

  824. <p>When <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> is initialized the main process is assigned a

  825. random string using <a class="reference internal" href="os.html#os.urandom" title="os.urandom"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.urandom()</span></code></a>.</p>

  826. <p>When a <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> object is created, it will inherit the

  827. authentication key of its parent process, although this may be changed by

  828. setting <a class="reference internal" href="#multiprocessing.Process.authkey" title="multiprocessing.Process.authkey"><code class="xref py py-attr docutils literal notranslate"><span class="pre">authkey</span></code></a> to another byte string.</p>

  829. <p>See <a class="reference internal" href="#multiprocessing-auth-keys"><span class="std std-ref">Authentication keys</span></a>.</p>

  830. </dd></dl>

  831. 

  832. <dl class="py attribute">

  833. <dt class="sig sig-object py" id="multiprocessing.Process.sentinel">

  834. <span class="sig-name descname"><span class="pre">sentinel</span></span><a class="headerlink" href="#multiprocessing.Process.sentinel" title="Link to this definition">¶</a></dt>

  835. <dd><p>A numeric handle of a system object which will become “ready” when

  836. the process ends.</p>

  837. <p>You can use this value if you want to wait on several events at

  838. once using <a class="reference internal" href="#multiprocessing.connection.wait" title="multiprocessing.connection.wait"><code class="xref py py-func docutils literal notranslate"><span class="pre">multiprocessing.connection.wait()</span></code></a>.  Otherwise

  839. calling <a class="reference internal" href="#multiprocessing.Process.join" title="multiprocessing.Process.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> is simpler.</p>

  840. <p>On Windows, this is an OS handle usable with the <code class="docutils literal notranslate"><span class="pre">WaitForSingleObject</span></code>

  841. and <code class="docutils literal notranslate"><span class="pre">WaitForMultipleObjects</span></code> family of API calls.  On POSIX, this is

  842. a file descriptor usable with primitives from the <a class="reference internal" href="select.html#module-select" title="select: Wait for I/O completion on multiple streams."><code class="xref py py-mod docutils literal notranslate"><span class="pre">select</span></code></a> module.</p>

  843. <div class="versionadded">

  844. <p><span class="versionmodified added">New in version 3.3.</span></p>

  845. </div>

  846. </dd></dl>

  847. 

  848. <dl class="py method">

  849. <dt class="sig sig-object py" id="multiprocessing.Process.terminate">

  850. <span class="sig-name descname"><span class="pre">terminate</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Process.terminate" title="Link to this definition">¶</a></dt>

  851. <dd><p>Terminate the process.  On POSIX this is done using the <a class="reference internal" href="signal.html#signal.SIGTERM" title="signal.SIGTERM"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGTERM</span></code></a> signal;

  852. on Windows <code class="xref c c-func docutils literal notranslate"><span class="pre">TerminateProcess()</span></code> is used.  Note that exit handlers and

  853. finally clauses, etc., will not be executed.</p>

  854. <p>Note that descendant processes of the process will <em>not</em> be terminated –

  855. they will simply become orphaned.</p>

  856. <div class="admonition warning">

  857. <p class="admonition-title">Warning</p>

  858. <p>If this method is used when the associated process is using a pipe or

  859. queue then the pipe or queue is liable to become corrupted and may

  860. become unusable by other process.  Similarly, if the process has

  861. acquired a lock or semaphore etc. then terminating it is liable to

  862. cause other processes to deadlock.</p>

  863. </div>

  864. </dd></dl>

  865. 

  866. <dl class="py method">

  867. <dt class="sig sig-object py" id="multiprocessing.Process.kill">

  868. <span class="sig-name descname"><span class="pre">kill</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Process.kill" title="Link to this definition">¶</a></dt>

  869. <dd><p>Same as <a class="reference internal" href="#multiprocessing.Process.terminate" title="multiprocessing.Process.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">terminate()</span></code></a> but using the <code class="docutils literal notranslate"><span class="pre">SIGKILL</span></code> signal on POSIX.</p>

  870. <div class="versionadded">

  871. <p><span class="versionmodified added">New in version 3.7.</span></p>

  872. </div>

  873. </dd></dl>

  874. 

  875. <dl class="py method">

  876. <dt class="sig sig-object py" id="multiprocessing.Process.close">

  877. <span class="sig-name descname"><span class="pre">close</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Process.close" title="Link to this definition">¶</a></dt>

  878. <dd><p>Close the <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> object, releasing all resources associated

  879. with it.  <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised if the underlying process

  880. is still running.  Once <a class="reference internal" href="#multiprocessing.Process.close" title="multiprocessing.Process.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> returns successfully, most

  881. other methods and attributes of the <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> object will

  882. raise <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a>.</p>

  883. <div class="versionadded">

  884. <p><span class="versionmodified added">New in version 3.7.</span></p>

  885. </div>

  886. </dd></dl>

  887. 

  888. <p>Note that the <a class="reference internal" href="#multiprocessing.Process.start" title="multiprocessing.Process.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">start()</span></code></a>, <a class="reference internal" href="#multiprocessing.Process.join" title="multiprocessing.Process.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a>, <a class="reference internal" href="#multiprocessing.Process.is_alive" title="multiprocessing.Process.is_alive"><code class="xref py py-meth docutils literal notranslate"><span class="pre">is_alive()</span></code></a>,

  889. <a class="reference internal" href="#multiprocessing.Process.terminate" title="multiprocessing.Process.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">terminate()</span></code></a> and <a class="reference internal" href="#multiprocessing.Process.exitcode" title="multiprocessing.Process.exitcode"><code class="xref py py-attr docutils literal notranslate"><span class="pre">exitcode</span></code></a> methods should only be called by

  890. the process that created the process object.</p>

  891. <p>Example usage of some of the methods of <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a>:</p>

  892. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">multiprocessing</span><span class="o">,</span> <span class="nn">time</span><span class="o">,</span> <span class="nn">signal</span>

  893. <span class="gp">&gt;&gt;&gt; </span><span class="n">mp_context</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">get_context</span><span class="p">(</span><span class="s1">&#39;spawn&#39;</span><span class="p">)</span>

  894. <span class="gp">&gt;&gt;&gt; </span><span class="n">p</span> <span class="o">=</span> <span class="n">mp_context</span><span class="o">.</span><span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="mi">1000</span><span class="p">,))</span>

  895. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="n">p</span><span class="o">.</span><span class="n">is_alive</span><span class="p">())</span>

  896. <span class="go">&lt;...Process ... initial&gt; False</span>

  897. <span class="gp">&gt;&gt;&gt; </span><span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  898. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="n">p</span><span class="o">.</span><span class="n">is_alive</span><span class="p">())</span>

  899. <span class="go">&lt;...Process ... started&gt; True</span>

  900. <span class="gp">&gt;&gt;&gt; </span><span class="n">p</span><span class="o">.</span><span class="n">terminate</span><span class="p">()</span>

  901. <span class="gp">&gt;&gt;&gt; </span><span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.1</span><span class="p">)</span>

  902. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="n">p</span><span class="o">.</span><span class="n">is_alive</span><span class="p">())</span>

  903. <span class="go">&lt;...Process ... stopped exitcode=-SIGTERM&gt; False</span>

  904. <span class="gp">&gt;&gt;&gt; </span><span class="n">p</span><span class="o">.</span><span class="n">exitcode</span> <span class="o">==</span> <span class="o">-</span><span class="n">signal</span><span class="o">.</span><span class="n">SIGTERM</span>

  905. <span class="go">True</span>

  906. </pre></div>

  907. </div>

  908. </dd></dl>

  909. 

  910. <dl class="py exception">

  911. <dt class="sig sig-object py" id="multiprocessing.ProcessError">

  912. <em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">ProcessError</span></span><a class="headerlink" href="#multiprocessing.ProcessError" title="Link to this definition">¶</a></dt>

  913. <dd><p>The base class of all <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> exceptions.</p>

  914. </dd></dl>

  915. 

  916. <dl class="py exception">

  917. <dt class="sig sig-object py" id="multiprocessing.BufferTooShort">

  918. <em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">BufferTooShort</span></span><a class="headerlink" href="#multiprocessing.BufferTooShort" title="Link to this definition">¶</a></dt>

  919. <dd><p>Exception raised by <code class="xref py py-meth docutils literal notranslate"><span class="pre">Connection.recv_bytes_into()</span></code> when the supplied

  920. buffer object is too small for the message read.</p>

  921. <p>If <code class="docutils literal notranslate"><span class="pre">e</span></code> is an instance of <a class="reference internal" href="#multiprocessing.BufferTooShort" title="multiprocessing.BufferTooShort"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BufferTooShort</span></code></a> then <code class="docutils literal notranslate"><span class="pre">e.args[0]</span></code> will give

  922. the message as a byte string.</p>

  923. </dd></dl>

  924. 

  925. <dl class="py exception">

  926. <dt class="sig sig-object py" id="multiprocessing.AuthenticationError">

  927. <em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">AuthenticationError</span></span><a class="headerlink" href="#multiprocessing.AuthenticationError" title="Link to this definition">¶</a></dt>

  928. <dd><p>Raised when there is an authentication error.</p>

  929. </dd></dl>

  930. 

  931. <dl class="py exception">

  932. <dt class="sig sig-object py" id="multiprocessing.TimeoutError">

  933. <em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">TimeoutError</span></span><a class="headerlink" href="#multiprocessing.TimeoutError" title="Link to this definition">¶</a></dt>

  934. <dd><p>Raised by methods with a timeout when the timeout expires.</p>

  935. </dd></dl>

  936. 

  937. </section>

  938. <section id="pipes-and-queues">

  939. <h3>Pipes and Queues<a class="headerlink" href="#pipes-and-queues" title="Link to this heading">¶</a></h3>

  940. <p>When using multiple processes, one generally uses message passing for

  941. communication between processes and avoids having to use any synchronization

  942. primitives like locks.</p>

  943. <p>For passing messages one can use <a class="reference internal" href="#multiprocessing.Pipe" title="multiprocessing.Pipe"><code class="xref py py-func docutils literal notranslate"><span class="pre">Pipe()</span></code></a> (for a connection between two

  944. processes) or a queue (which allows multiple producers and consumers).</p>

  945. <p>The <a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a>, <a class="reference internal" href="#multiprocessing.SimpleQueue" title="multiprocessing.SimpleQueue"><code class="xref py py-class docutils literal notranslate"><span class="pre">SimpleQueue</span></code></a> and <a class="reference internal" href="#multiprocessing.JoinableQueue" title="multiprocessing.JoinableQueue"><code class="xref py py-class docutils literal notranslate"><span class="pre">JoinableQueue</span></code></a> types

  946. are multi-producer, multi-consumer <abbr title="first-in, first-out">FIFO</abbr>

  947. queues modelled on the <a class="reference internal" href="queue.html#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">queue.Queue</span></code></a> class in the

  948. standard library.  They differ in that <a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a> lacks the

  949. <a class="reference internal" href="queue.html#queue.Queue.task_done" title="queue.Queue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> and <a class="reference internal" href="queue.html#queue.Queue.join" title="queue.Queue.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> methods introduced

  950. into Python 2.5’s <a class="reference internal" href="queue.html#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">queue.Queue</span></code></a> class.</p>

  951. <p>If you use <a class="reference internal" href="#multiprocessing.JoinableQueue" title="multiprocessing.JoinableQueue"><code class="xref py py-class docutils literal notranslate"><span class="pre">JoinableQueue</span></code></a> then you <strong>must</strong> call

  952. <a class="reference internal" href="#multiprocessing.JoinableQueue.task_done" title="multiprocessing.JoinableQueue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">JoinableQueue.task_done()</span></code></a> for each task removed from the queue or else the

  953. semaphore used to count the number of unfinished tasks may eventually overflow,

  954. raising an exception.</p>

  955. <p>Note that one can also create a shared queue by using a manager object – see

  956. <a class="reference internal" href="#multiprocessing-managers"><span class="std std-ref">Managers</span></a>.</p>

  957. <div class="admonition note">

  958. <p class="admonition-title">Note</p>

  959. <p><a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> uses the usual <a class="reference internal" href="queue.html#queue.Empty" title="queue.Empty"><code class="xref py py-exc docutils literal notranslate"><span class="pre">queue.Empty</span></code></a> and

  960. <a class="reference internal" href="queue.html#queue.Full" title="queue.Full"><code class="xref py py-exc docutils literal notranslate"><span class="pre">queue.Full</span></code></a> exceptions to signal a timeout.  They are not available in

  961. the <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> namespace so you need to import them from

  962. <a class="reference internal" href="queue.html#module-queue" title="queue: A synchronized queue class."><code class="xref py py-mod docutils literal notranslate"><span class="pre">queue</span></code></a>.</p>

  963. </div>

  964. <div class="admonition note">

  965. <p class="admonition-title">Note</p>

  966. <p>When an object is put on a queue, the object is pickled and a

  967. background thread later flushes the pickled data to an underlying

  968. pipe.  This has some consequences which are a little surprising,

  969. but should not cause any practical difficulties – if they really

  970. bother you then you can instead use a queue created with a

  971. <a class="reference internal" href="#multiprocessing-managers"><span class="std std-ref">manager</span></a>.</p>

  972. <ol class="arabic simple">

  973. <li><p>After putting an object on an empty queue there may be an

  974. infinitesimal delay before the queue’s <a class="reference internal" href="#multiprocessing.Queue.empty" title="multiprocessing.Queue.empty"><code class="xref py py-meth docutils literal notranslate"><span class="pre">empty()</span></code></a>

  975. method returns <a class="reference internal" href="constants.html#False" title="False"><code class="xref py py-const docutils literal notranslate"><span class="pre">False</span></code></a> and <a class="reference internal" href="#multiprocessing.Queue.get_nowait" title="multiprocessing.Queue.get_nowait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get_nowait()</span></code></a> can

  976. return without raising <a class="reference internal" href="queue.html#queue.Empty" title="queue.Empty"><code class="xref py py-exc docutils literal notranslate"><span class="pre">queue.Empty</span></code></a>.</p></li>

  977. <li><p>If multiple processes are enqueuing objects, it is possible for

  978. the objects to be received at the other end out-of-order.

  979. However, objects enqueued by the same process will always be in

  980. the expected order with respect to each other.</p></li>

  981. </ol>

  982. </div>

  983. <div class="admonition warning">

  984. <p class="admonition-title">Warning</p>

  985. <p>If a process is killed using <a class="reference internal" href="#multiprocessing.Process.terminate" title="multiprocessing.Process.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Process.terminate()</span></code></a> or <a class="reference internal" href="os.html#os.kill" title="os.kill"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.kill()</span></code></a>

  986. while it is trying to use a <a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a>, then the data in the queue is

  987. likely to become corrupted.  This may cause any other process to get an

  988. exception when it tries to use the queue later on.</p>

  989. </div>

  990. <div class="admonition warning">

  991. <p class="admonition-title">Warning</p>

  992. <p>As mentioned above, if a child process has put items on a queue (and it has

  993. not used <a class="reference internal" href="#multiprocessing.Queue.cancel_join_thread" title="multiprocessing.Queue.cancel_join_thread"><code class="xref py py-meth docutils literal notranslate"><span class="pre">JoinableQueue.cancel_join_thread</span></code></a>), then that process will

  994. not terminate until all buffered items have been flushed to the pipe.</p>

  995. <p>This means that if you try joining that process you may get a deadlock unless

  996. you are sure that all items which have been put on the queue have been

  997. consumed.  Similarly, if the child process is non-daemonic then the parent

  998. process may hang on exit when it tries to join all its non-daemonic children.</p>

  999. <p>Note that a queue created using a manager does not have this issue.  See

  1000. <a class="reference internal" href="#multiprocessing-programming"><span class="std std-ref">Programming guidelines</span></a>.</p>

  1001. </div>

  1002. <p>For an example of the usage of queues for interprocess communication see

  1003. <a class="reference internal" href="#multiprocessing-examples"><span class="std std-ref">Examples</span></a>.</p>

  1004. <dl class="py function">

  1005. <dt class="sig sig-object py" id="multiprocessing.Pipe">

  1006. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Pipe</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">duplex</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Pipe" title="Link to this definition">¶</a></dt>

  1007. <dd><p>Returns a pair <code class="docutils literal notranslate"><span class="pre">(conn1,</span> <span class="pre">conn2)</span></code> of

  1008. <a class="reference internal" href="#multiprocessing.connection.Connection" title="multiprocessing.connection.Connection"><code class="xref py py-class docutils literal notranslate"><span class="pre">Connection</span></code></a> objects representing the

  1009. ends of a pipe.</p>

  1010. <p>If <em>duplex</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code> (the default) then the pipe is bidirectional.  If

  1011. <em>duplex</em> is <code class="docutils literal notranslate"><span class="pre">False</span></code> then the pipe is unidirectional: <code class="docutils literal notranslate"><span class="pre">conn1</span></code> can only be

  1012. used for receiving messages and <code class="docutils literal notranslate"><span class="pre">conn2</span></code> can only be used for sending

  1013. messages.</p>

  1014. </dd></dl>

  1015. 

  1016. <dl class="py class">

  1017. <dt class="sig sig-object py" id="multiprocessing.Queue">

  1018. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Queue</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">maxsize</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue" title="Link to this definition">¶</a></dt>

  1019. <dd><p>Returns a process shared queue implemented using a pipe and a few

  1020. locks/semaphores.  When a process first puts an item on the queue a feeder

  1021. thread is started which transfers objects from a buffer into the pipe.</p>

  1022. <p>The usual <a class="reference internal" href="queue.html#queue.Empty" title="queue.Empty"><code class="xref py py-exc docutils literal notranslate"><span class="pre">queue.Empty</span></code></a> and <a class="reference internal" href="queue.html#queue.Full" title="queue.Full"><code class="xref py py-exc docutils literal notranslate"><span class="pre">queue.Full</span></code></a> exceptions from the

  1023. standard library’s <a class="reference internal" href="queue.html#module-queue" title="queue: A synchronized queue class."><code class="xref py py-mod docutils literal notranslate"><span class="pre">queue</span></code></a> module are raised to signal timeouts.</p>

  1024. <p><a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a> implements all the methods of <a class="reference internal" href="queue.html#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">queue.Queue</span></code></a> except for

  1025. <a class="reference internal" href="queue.html#queue.Queue.task_done" title="queue.Queue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> and <a class="reference internal" href="queue.html#queue.Queue.join" title="queue.Queue.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a>.</p>

  1026. <dl class="py method">

  1027. <dt class="sig sig-object py" id="multiprocessing.Queue.qsize">

  1028. <span class="sig-name descname"><span class="pre">qsize</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.qsize" title="Link to this definition">¶</a></dt>

  1029. <dd><p>Return the approximate size of the queue.  Because of

  1030. multithreading/multiprocessing semantics, this number is not reliable.</p>

  1031. <p>Note that this may raise <a class="reference internal" href="exceptions.html#NotImplementedError" title="NotImplementedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NotImplementedError</span></code></a> on platforms like

  1032. macOS where <code class="docutils literal notranslate"><span class="pre">sem_getvalue()</span></code> is not implemented.</p>

  1033. </dd></dl>

  1034. 

  1035. <dl class="py method">

  1036. <dt class="sig sig-object py" id="multiprocessing.Queue.empty">

  1037. <span class="sig-name descname"><span class="pre">empty</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.empty" title="Link to this definition">¶</a></dt>

  1038. <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the queue is empty, <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.  Because of

  1039. multithreading/multiprocessing semantics, this is not reliable.</p>

  1040. </dd></dl>

  1041. 

  1042. <dl class="py method">

  1043. <dt class="sig sig-object py" id="multiprocessing.Queue.full">

  1044. <span class="sig-name descname"><span class="pre">full</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.full" title="Link to this definition">¶</a></dt>

  1045. <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the queue is full, <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.  Because of

  1046. multithreading/multiprocessing semantics, this is not reliable.</p>

  1047. </dd></dl>

  1048. 

  1049. <dl class="py method">

  1050. <dt class="sig sig-object py" id="multiprocessing.Queue.put">

  1051. <span class="sig-name descname"><span class="pre">put</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">obj</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">block</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.put" title="Link to this definition">¶</a></dt>

  1052. <dd><p>Put obj into the queue.  If the optional argument <em>block</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code>

  1053. (the default) and <em>timeout</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> (the default), block if necessary until

  1054. a free slot is available.  If <em>timeout</em> is a positive number, it blocks at

  1055. most <em>timeout</em> seconds and raises the <a class="reference internal" href="queue.html#queue.Full" title="queue.Full"><code class="xref py py-exc docutils literal notranslate"><span class="pre">queue.Full</span></code></a> exception if no

  1056. free slot was available within that time.  Otherwise (<em>block</em> is

  1057. <code class="docutils literal notranslate"><span class="pre">False</span></code>), put an item on the queue if a free slot is immediately

  1058. available, else raise the <a class="reference internal" href="queue.html#queue.Full" title="queue.Full"><code class="xref py py-exc docutils literal notranslate"><span class="pre">queue.Full</span></code></a> exception (<em>timeout</em> is

  1059. ignored in that case).</p>

  1060. <div class="versionchanged">

  1061. <p><span class="versionmodified changed">Changed in version 3.8: </span>If the queue is closed, <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised instead of

  1062. <a class="reference internal" href="exceptions.html#AssertionError" title="AssertionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AssertionError</span></code></a>.</p>

  1063. </div>

  1064. </dd></dl>

  1065. 

  1066. <dl class="py method">

  1067. <dt class="sig sig-object py" id="multiprocessing.Queue.put_nowait">

  1068. <span class="sig-name descname"><span class="pre">put_nowait</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">obj</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.put_nowait" title="Link to this definition">¶</a></dt>

  1069. <dd><p>Equivalent to <code class="docutils literal notranslate"><span class="pre">put(obj,</span> <span class="pre">False)</span></code>.</p>

  1070. </dd></dl>

  1071. 

  1072. <dl class="py method">

  1073. <dt class="sig sig-object py" id="multiprocessing.Queue.get">

  1074. <span class="sig-name descname"><span class="pre">get</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">block</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.get" title="Link to this definition">¶</a></dt>

  1075. <dd><p>Remove and return an item from the queue.  If optional args <em>block</em> is

  1076. <code class="docutils literal notranslate"><span class="pre">True</span></code> (the default) and <em>timeout</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> (the default), block if

  1077. necessary until an item is available.  If <em>timeout</em> is a positive number,

  1078. it blocks at most <em>timeout</em> seconds and raises the <a class="reference internal" href="queue.html#queue.Empty" title="queue.Empty"><code class="xref py py-exc docutils literal notranslate"><span class="pre">queue.Empty</span></code></a>

  1079. exception if no item was available within that time.  Otherwise (block is

  1080. <code class="docutils literal notranslate"><span class="pre">False</span></code>), return an item if one is immediately available, else raise the

  1081. <a class="reference internal" href="queue.html#queue.Empty" title="queue.Empty"><code class="xref py py-exc docutils literal notranslate"><span class="pre">queue.Empty</span></code></a> exception (<em>timeout</em> is ignored in that case).</p>

  1082. <div class="versionchanged">

  1083. <p><span class="versionmodified changed">Changed in version 3.8: </span>If the queue is closed, <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised instead of

  1084. <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a>.</p>

  1085. </div>

  1086. </dd></dl>

  1087. 

  1088. <dl class="py method">

  1089. <dt class="sig sig-object py" id="multiprocessing.Queue.get_nowait">

  1090. <span class="sig-name descname"><span class="pre">get_nowait</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.get_nowait" title="Link to this definition">¶</a></dt>

  1091. <dd><p>Equivalent to <code class="docutils literal notranslate"><span class="pre">get(False)</span></code>.</p>

  1092. </dd></dl>

  1093. 

  1094. <p><a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">multiprocessing.Queue</span></code></a> has a few additional methods not found in

  1095. <a class="reference internal" href="queue.html#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">queue.Queue</span></code></a>.  These methods are usually unnecessary for most

  1096. code:</p>

  1097. <dl class="py method">

  1098. <dt class="sig sig-object py" id="multiprocessing.Queue.close">

  1099. <span class="sig-name descname"><span class="pre">close</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.close" title="Link to this definition">¶</a></dt>

  1100. <dd><p>Indicate that no more data will be put on this queue by the current

  1101. process.  The background thread will quit once it has flushed all buffered

  1102. data to the pipe.  This is called automatically when the queue is garbage

  1103. collected.</p>

  1104. </dd></dl>

  1105. 

  1106. <dl class="py method">

  1107. <dt class="sig sig-object py" id="multiprocessing.Queue.join_thread">

  1108. <span class="sig-name descname"><span class="pre">join_thread</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.join_thread" title="Link to this definition">¶</a></dt>

  1109. <dd><p>Join the background thread.  This can only be used after <a class="reference internal" href="#multiprocessing.Queue.close" title="multiprocessing.Queue.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> has

  1110. been called.  It blocks until the background thread exits, ensuring that

  1111. all data in the buffer has been flushed to the pipe.</p>

  1112. <p>By default if a process is not the creator of the queue then on exit it

  1113. will attempt to join the queue’s background thread.  The process can call

  1114. <a class="reference internal" href="#multiprocessing.Queue.cancel_join_thread" title="multiprocessing.Queue.cancel_join_thread"><code class="xref py py-meth docutils literal notranslate"><span class="pre">cancel_join_thread()</span></code></a> to make <a class="reference internal" href="#multiprocessing.Queue.join_thread" title="multiprocessing.Queue.join_thread"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join_thread()</span></code></a> do nothing.</p>

  1115. </dd></dl>

  1116. 

  1117. <dl class="py method">

  1118. <dt class="sig sig-object py" id="multiprocessing.Queue.cancel_join_thread">

  1119. <span class="sig-name descname"><span class="pre">cancel_join_thread</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Queue.cancel_join_thread" title="Link to this definition">¶</a></dt>

  1120. <dd><p>Prevent <a class="reference internal" href="#multiprocessing.Queue.join_thread" title="multiprocessing.Queue.join_thread"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join_thread()</span></code></a> from blocking.  In particular, this prevents

  1121. the background thread from being joined automatically when the process

  1122. exits – see <a class="reference internal" href="#multiprocessing.Queue.join_thread" title="multiprocessing.Queue.join_thread"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join_thread()</span></code></a>.</p>

  1123. <p>A better name for this method might be

  1124. <code class="docutils literal notranslate"><span class="pre">allow_exit_without_flush()</span></code>.  It is likely to cause enqueued

  1125. data to be lost, and you almost certainly will not need to use it.

  1126. It is really only there if you need the current process to exit

  1127. immediately without waiting to flush enqueued data to the

  1128. underlying pipe, and you don’t care about lost data.</p>

  1129. </dd></dl>

  1130. 

  1131. <div class="admonition note">

  1132. <p class="admonition-title">Note</p>

  1133. <p>This class’s functionality requires a functioning shared semaphore

  1134. implementation on the host operating system. Without one, the

  1135. functionality in this class will be disabled, and attempts to

  1136. instantiate a <a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a> will result in an <a class="reference internal" href="exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a>. See

  1137. <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=3770">bpo-3770</a> for additional information.  The same holds true for any

  1138. of the specialized queue types listed below.</p>

  1139. </div>

  1140. </dd></dl>

  1141. 

  1142. <dl class="py class">

  1143. <dt class="sig sig-object py" id="multiprocessing.SimpleQueue">

  1144. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">SimpleQueue</span></span><a class="headerlink" href="#multiprocessing.SimpleQueue" title="Link to this definition">¶</a></dt>

  1145. <dd><p>It is a simplified <a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a> type, very close to a locked <a class="reference internal" href="#multiprocessing.Pipe" title="multiprocessing.Pipe"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pipe</span></code></a>.</p>

  1146. <dl class="py method">

  1147. <dt class="sig sig-object py" id="multiprocessing.SimpleQueue.close">

  1148. <span class="sig-name descname"><span class="pre">close</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.SimpleQueue.close" title="Link to this definition">¶</a></dt>

  1149. <dd><p>Close the queue: release internal resources.</p>

  1150. <p>A queue must not be used anymore after it is closed. For example,

  1151. <a class="reference internal" href="#multiprocessing.SimpleQueue.get" title="multiprocessing.SimpleQueue.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code></a>, <a class="reference internal" href="#multiprocessing.SimpleQueue.put" title="multiprocessing.SimpleQueue.put"><code class="xref py py-meth docutils literal notranslate"><span class="pre">put()</span></code></a> and <a class="reference internal" href="#multiprocessing.SimpleQueue.empty" title="multiprocessing.SimpleQueue.empty"><code class="xref py py-meth docutils literal notranslate"><span class="pre">empty()</span></code></a> methods must no longer be

  1152. called.</p>

  1153. <div class="versionadded">

  1154. <p><span class="versionmodified added">New in version 3.9.</span></p>

  1155. </div>

  1156. </dd></dl>

  1157. 

  1158. <dl class="py method">

  1159. <dt class="sig sig-object py" id="multiprocessing.SimpleQueue.empty">

  1160. <span class="sig-name descname"><span class="pre">empty</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.SimpleQueue.empty" title="Link to this definition">¶</a></dt>

  1161. <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the queue is empty, <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.</p>

  1162. </dd></dl>

  1163. 

  1164. <dl class="py method">

  1165. <dt class="sig sig-object py" id="multiprocessing.SimpleQueue.get">

  1166. <span class="sig-name descname"><span class="pre">get</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.SimpleQueue.get" title="Link to this definition">¶</a></dt>

  1167. <dd><p>Remove and return an item from the queue.</p>

  1168. </dd></dl>

  1169. 

  1170. <dl class="py method">

  1171. <dt class="sig sig-object py" id="multiprocessing.SimpleQueue.put">

  1172. <span class="sig-name descname"><span class="pre">put</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">item</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.SimpleQueue.put" title="Link to this definition">¶</a></dt>

  1173. <dd><p>Put <em>item</em> into the queue.</p>

  1174. </dd></dl>

  1175. 

  1176. </dd></dl>

  1177. 

  1178. <dl class="py class">

  1179. <dt class="sig sig-object py" id="multiprocessing.JoinableQueue">

  1180. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">JoinableQueue</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">maxsize</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.JoinableQueue" title="Link to this definition">¶</a></dt>

  1181. <dd><p><a class="reference internal" href="#multiprocessing.JoinableQueue" title="multiprocessing.JoinableQueue"><code class="xref py py-class docutils literal notranslate"><span class="pre">JoinableQueue</span></code></a>, a <a class="reference internal" href="#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a> subclass, is a queue which

  1182. additionally has <a class="reference internal" href="#multiprocessing.JoinableQueue.task_done" title="multiprocessing.JoinableQueue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> and <a class="reference internal" href="#multiprocessing.JoinableQueue.join" title="multiprocessing.JoinableQueue.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> methods.</p>

  1183. <dl class="py method">

  1184. <dt class="sig sig-object py" id="multiprocessing.JoinableQueue.task_done">

  1185. <span class="sig-name descname"><span class="pre">task_done</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.JoinableQueue.task_done" title="Link to this definition">¶</a></dt>

  1186. <dd><p>Indicate that a formerly enqueued task is complete. Used by queue

  1187. consumers.  For each <a class="reference internal" href="#multiprocessing.Queue.get" title="multiprocessing.Queue.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code></a> used to fetch a task, a subsequent

  1188. call to <a class="reference internal" href="#multiprocessing.JoinableQueue.task_done" title="multiprocessing.JoinableQueue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> tells the queue that the processing on the task

  1189. is complete.</p>

  1190. <p>If a <a class="reference internal" href="queue.html#queue.Queue.join" title="queue.Queue.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> is currently blocking, it will resume when all

  1191. items have been processed (meaning that a <a class="reference internal" href="#multiprocessing.JoinableQueue.task_done" title="multiprocessing.JoinableQueue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> call was

  1192. received for every item that had been <a class="reference internal" href="#multiprocessing.Queue.put" title="multiprocessing.Queue.put"><code class="xref py py-meth docutils literal notranslate"><span class="pre">put()</span></code></a> into the queue).</p>

  1193. <p>Raises a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> if called more times than there were items

  1194. placed in the queue.</p>

  1195. </dd></dl>

  1196. 

  1197. <dl class="py method">

  1198. <dt class="sig sig-object py" id="multiprocessing.JoinableQueue.join">

  1199. <span class="sig-name descname"><span class="pre">join</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.JoinableQueue.join" title="Link to this definition">¶</a></dt>

  1200. <dd><p>Block until all items in the queue have been gotten and processed.</p>

  1201. <p>The count of unfinished tasks goes up whenever an item is added to the

  1202. queue.  The count goes down whenever a consumer calls

  1203. <a class="reference internal" href="#multiprocessing.JoinableQueue.task_done" title="multiprocessing.JoinableQueue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> to indicate that the item was retrieved and all work on

  1204. it is complete.  When the count of unfinished tasks drops to zero,

  1205. <a class="reference internal" href="queue.html#queue.Queue.join" title="queue.Queue.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> unblocks.</p>

  1206. </dd></dl>

  1207. 

  1208. </dd></dl>

  1209. 

  1210. </section>

  1211. <section id="miscellaneous">

  1212. <h3>Miscellaneous<a class="headerlink" href="#miscellaneous" title="Link to this heading">¶</a></h3>

  1213. <dl class="py function">

  1214. <dt class="sig sig-object py" id="multiprocessing.active_children">

  1215. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">active_children</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.active_children" title="Link to this definition">¶</a></dt>

  1216. <dd><p>Return list of all live children of the current process.</p>

  1217. <p>Calling this has the side effect of “joining” any processes which have

  1218. already finished.</p>

  1219. </dd></dl>

  1220. 

  1221. <dl class="py function">

  1222. <dt class="sig sig-object py" id="multiprocessing.cpu_count">

  1223. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">cpu_count</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.cpu_count" title="Link to this definition">¶</a></dt>

  1224. <dd><p>Return the number of CPUs in the system.</p>

  1225. <p>This number is not equivalent to the number of CPUs the current process can

  1226. use.  The number of usable CPUs can be obtained with

  1227. <code class="docutils literal notranslate"><span class="pre">len(os.sched_getaffinity(0))</span></code></p>

  1228. <p>When the number of CPUs cannot be determined a <a class="reference internal" href="exceptions.html#NotImplementedError" title="NotImplementedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NotImplementedError</span></code></a>

  1229. is raised.</p>

  1230. <div class="admonition seealso">

  1231. <p class="admonition-title">See also</p>

  1232. <p><a class="reference internal" href="os.html#os.cpu_count" title="os.cpu_count"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.cpu_count()</span></code></a></p>

  1233. </div>

  1234. </dd></dl>

  1235. 

  1236. <dl class="py function">

  1237. <dt class="sig sig-object py" id="multiprocessing.current_process">

  1238. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">current_process</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.current_process" title="Link to this definition">¶</a></dt>

  1239. <dd><p>Return the <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> object corresponding to the current process.</p>

  1240. <p>An analogue of <a class="reference internal" href="threading.html#threading.current_thread" title="threading.current_thread"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.current_thread()</span></code></a>.</p>

  1241. </dd></dl>

  1242. 

  1243. <dl class="py function">

  1244. <dt class="sig sig-object py" id="multiprocessing.parent_process">

  1245. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">parent_process</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.parent_process" title="Link to this definition">¶</a></dt>

  1246. <dd><p>Return the <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> object corresponding to the parent process of

  1247. the <a class="reference internal" href="#multiprocessing.current_process" title="multiprocessing.current_process"><code class="xref py py-func docutils literal notranslate"><span class="pre">current_process()</span></code></a>. For the main process, <code class="docutils literal notranslate"><span class="pre">parent_process</span></code> will

  1248. be <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>

  1249. <div class="versionadded">

  1250. <p><span class="versionmodified added">New in version 3.8.</span></p>

  1251. </div>

  1252. </dd></dl>

  1253. 

  1254. <dl class="py function">

  1255. <dt class="sig sig-object py" id="multiprocessing.freeze_support">

  1256. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">freeze_support</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.freeze_support" title="Link to this definition">¶</a></dt>

  1257. <dd><p>Add support for when a program which uses <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> has been

  1258. frozen to produce a Windows executable.  (Has been tested with <strong>py2exe</strong>,

  1259. <strong>PyInstaller</strong> and <strong>cx_Freeze</strong>.)</p>

  1260. <p>One needs to call this function straight after the <code class="docutils literal notranslate"><span class="pre">if</span> <span class="pre">__name__</span> <span class="pre">==</span>

  1261. <span class="pre">'__main__'</span></code> line of the main module.  For example:</p>

  1262. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">freeze_support</span>

  1263. 

  1264. <span class="k">def</span> <span class="nf">f</span><span class="p">():</span>

  1265.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;hello world!&#39;</span><span class="p">)</span>

  1266. 

  1267. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  1268.     <span class="n">freeze_support</span><span class="p">()</span>

  1269.     <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">)</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  1270. </pre></div>

  1271. </div>

  1272. <p>If the <code class="docutils literal notranslate"><span class="pre">freeze_support()</span></code> line is omitted then trying to run the frozen

  1273. executable will raise <a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a>.</p>

  1274. <p>Calling <code class="docutils literal notranslate"><span class="pre">freeze_support()</span></code> has no effect when invoked on any operating

  1275. system other than Windows.  In addition, if the module is being run

  1276. normally by the Python interpreter on Windows (the program has not been

  1277. frozen), then <code class="docutils literal notranslate"><span class="pre">freeze_support()</span></code> has no effect.</p>

  1278. </dd></dl>

  1279. 

  1280. <dl class="py function">

  1281. <dt class="sig sig-object py" id="multiprocessing.get_all_start_methods">

  1282. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">get_all_start_methods</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.get_all_start_methods" title="Link to this definition">¶</a></dt>

  1283. <dd><p>Returns a list of the supported start methods, the first of which

  1284. is the default.  The possible start methods are <code class="docutils literal notranslate"><span class="pre">'fork'</span></code>,

  1285. <code class="docutils literal notranslate"><span class="pre">'spawn'</span></code> and <code class="docutils literal notranslate"><span class="pre">'forkserver'</span></code>.  Not all platforms support all

  1286. methods.  See <a class="reference internal" href="#multiprocessing-start-methods"><span class="std std-ref">Contexts and start methods</span></a>.</p>

  1287. <div class="versionadded">

  1288. <p><span class="versionmodified added">New in version 3.4.</span></p>

  1289. </div>

  1290. </dd></dl>

  1291. 

  1292. <dl class="py function">

  1293. <dt class="sig sig-object py" id="multiprocessing.get_context">

  1294. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">get_context</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">method</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.get_context" title="Link to this definition">¶</a></dt>

  1295. <dd><p>Return a context object which has the same attributes as the

  1296. <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> module.</p>

  1297. <p>If <em>method</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> then the default context is returned.

  1298. Otherwise <em>method</em> should be <code class="docutils literal notranslate"><span class="pre">'fork'</span></code>, <code class="docutils literal notranslate"><span class="pre">'spawn'</span></code>,

  1299. <code class="docutils literal notranslate"><span class="pre">'forkserver'</span></code>.  <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised if the specified

  1300. start method is not available.  See <a class="reference internal" href="#multiprocessing-start-methods"><span class="std std-ref">Contexts and start methods</span></a>.</p>

  1301. <div class="versionadded">

  1302. <p><span class="versionmodified added">New in version 3.4.</span></p>

  1303. </div>

  1304. </dd></dl>

  1305. 

  1306. <dl class="py function">

  1307. <dt class="sig sig-object py" id="multiprocessing.get_start_method">

  1308. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">get_start_method</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">allow_none</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">False</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.get_start_method" title="Link to this definition">¶</a></dt>

  1309. <dd><p>Return the name of start method used for starting processes.</p>

  1310. <p>If the start method has not been fixed and <em>allow_none</em> is false,

  1311. then the start method is fixed to the default and the name is

  1312. returned.  If the start method has not been fixed and <em>allow_none</em>

  1313. is true then <code class="docutils literal notranslate"><span class="pre">None</span></code> is returned.</p>

  1314. <p>The return value can be <code class="docutils literal notranslate"><span class="pre">'fork'</span></code>, <code class="docutils literal notranslate"><span class="pre">'spawn'</span></code>, <code class="docutils literal notranslate"><span class="pre">'forkserver'</span></code>

  1315. or <code class="docutils literal notranslate"><span class="pre">None</span></code>.  See <a class="reference internal" href="#multiprocessing-start-methods"><span class="std std-ref">Contexts and start methods</span></a>.</p>

  1316. <div class="versionadded">

  1317. <p><span class="versionmodified added">New in version 3.4.</span></p>

  1318. </div>

  1319. <div class="versionchanged">

  1320. <p><span class="versionmodified changed">Changed in version 3.8: </span>On macOS, the <em>spawn</em> start method is now the default.  The <em>fork</em> start

  1321. method should be considered unsafe as it can lead to crashes of the

  1322. subprocess. See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33725">bpo-33725</a>.</p>

  1323. </div>

  1324. </dd></dl>

  1325. 

  1326. <dl class="py function">

  1327. <dt class="sig sig-object py" id="multiprocessing.set_executable">

  1328. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">set_executable</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">executable</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.set_executable" title="Link to this definition">¶</a></dt>

  1329. <dd><p>Set the path of the Python interpreter to use when starting a child process.

  1330. (By default <a class="reference internal" href="sys.html#sys.executable" title="sys.executable"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.executable</span></code></a> is used).  Embedders will probably need to

  1331. do some thing like</p>

  1332. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">set_executable</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">exec_prefix</span><span class="p">,</span> <span class="s1">&#39;pythonw.exe&#39;</span><span class="p">))</span>

  1333. </pre></div>

  1334. </div>

  1335. <p>before they can create child processes.</p>

  1336. <div class="versionchanged">

  1337. <p><span class="versionmodified changed">Changed in version 3.4: </span>Now supported on POSIX when the <code class="docutils literal notranslate"><span class="pre">'spawn'</span></code> start method is used.</p>

  1338. </div>

  1339. <div class="versionchanged">

  1340. <p><span class="versionmodified changed">Changed in version 3.11: </span>Accepts a <a class="reference internal" href="../glossary.html#term-path-like-object"><span class="xref std std-term">path-like object</span></a>.</p>

  1341. </div>

  1342. </dd></dl>

  1343. 

  1344. <dl class="py function">

  1345. <dt class="sig sig-object py" id="multiprocessing.set_forkserver_preload">

  1346. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">set_forkserver_preload</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">module_names</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.set_forkserver_preload" title="Link to this definition">¶</a></dt>

  1347. <dd><p>Set a list of module names for the forkserver main process to attempt to

  1348. import so that their already imported state is inherited by forked

  1349. processes. Any <a class="reference internal" href="exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a> when doing so is silently ignored.

  1350. This can be used as a performance enhancement to avoid repeated work

  1351. in every process.</p>

  1352. <p>For this to work, it must be called before the forkserver process has been

  1353. launched (before creating a <code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code> or starting a <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a>).</p>

  1354. <p>Only meaningful when using the <code class="docutils literal notranslate"><span class="pre">'forkserver'</span></code> start method.

  1355. See <a class="reference internal" href="#multiprocessing-start-methods"><span class="std std-ref">Contexts and start methods</span></a>.</p>

  1356. <div class="versionadded">

  1357. <p><span class="versionmodified added">New in version 3.4.</span></p>

  1358. </div>

  1359. </dd></dl>

  1360. 

  1361. <dl class="py function">

  1362. <dt class="sig sig-object py" id="multiprocessing.set_start_method">

  1363. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">set_start_method</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">method</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">force</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">False</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.set_start_method" title="Link to this definition">¶</a></dt>

  1364. <dd><p>Set the method which should be used to start child processes.

  1365. The <em>method</em> argument can be <code class="docutils literal notranslate"><span class="pre">'fork'</span></code>, <code class="docutils literal notranslate"><span class="pre">'spawn'</span></code> or <code class="docutils literal notranslate"><span class="pre">'forkserver'</span></code>.

  1366. Raises <a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a> if the start method has already been set and <em>force</em>

  1367. is not <code class="docutils literal notranslate"><span class="pre">True</span></code>.  If <em>method</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> and <em>force</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code> then the start

  1368. method is set to <code class="docutils literal notranslate"><span class="pre">None</span></code>.  If <em>method</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> and <em>force</em> is <code class="docutils literal notranslate"><span class="pre">False</span></code>

  1369. then the context is set to the default context.</p>

  1370. <p>Note that this should be called at most once, and it should be

  1371. protected inside the <code class="docutils literal notranslate"><span class="pre">if</span> <span class="pre">__name__</span> <span class="pre">==</span> <span class="pre">'__main__'</span></code> clause of the

  1372. main module.</p>

  1373. <p>See <a class="reference internal" href="#multiprocessing-start-methods"><span class="std std-ref">Contexts and start methods</span></a>.</p>

  1374. <div class="versionadded">

  1375. <p><span class="versionmodified added">New in version 3.4.</span></p>

  1376. </div>

  1377. </dd></dl>

  1378. 

  1379. <div class="admonition note">

  1380. <p class="admonition-title">Note</p>

  1381. <p><a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> contains no analogues of

  1382. <a class="reference internal" href="threading.html#threading.active_count" title="threading.active_count"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.active_count()</span></code></a>, <a class="reference internal" href="threading.html#threading.enumerate" title="threading.enumerate"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.enumerate()</span></code></a>,

  1383. <a class="reference internal" href="threading.html#threading.settrace" title="threading.settrace"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.settrace()</span></code></a>, <a class="reference internal" href="threading.html#threading.setprofile" title="threading.setprofile"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.setprofile()</span></code></a>,

  1384. <a class="reference internal" href="threading.html#threading.Timer" title="threading.Timer"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Timer</span></code></a>, or <a class="reference internal" href="threading.html#threading.local" title="threading.local"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.local</span></code></a>.</p>

  1385. </div>

  1386. </section>

  1387. <section id="connection-objects">

  1388. <h3>Connection Objects<a class="headerlink" href="#connection-objects" title="Link to this heading">¶</a></h3>

  1389. <p>Connection objects allow the sending and receiving of picklable objects or

  1390. strings.  They can be thought of as message oriented connected sockets.</p>

  1391. <p>Connection objects are usually created using

  1392. <a class="reference internal" href="#multiprocessing.Pipe" title="multiprocessing.Pipe"><code class="xref py py-func docutils literal notranslate"><span class="pre">Pipe</span></code></a> – see also

  1393. <a class="reference internal" href="#multiprocessing-listeners-clients"><span class="std std-ref">Listeners and Clients</span></a>.</p>

  1394. <dl class="py class">

  1395. <dt class="sig sig-object py" id="multiprocessing.connection.Connection">

  1396. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.connection.</span></span><span class="sig-name descname"><span class="pre">Connection</span></span><a class="headerlink" href="#multiprocessing.connection.Connection" title="Link to this definition">¶</a></dt>

  1397. <dd><dl class="py method">

  1398. <dt class="sig sig-object py" id="multiprocessing.connection.Connection.send">

  1399. <span class="sig-name descname"><span class="pre">send</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">obj</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Connection.send" title="Link to this definition">¶</a></dt>

  1400. <dd><p>Send an object to the other end of the connection which should be read

  1401. using <a class="reference internal" href="#multiprocessing.connection.Connection.recv" title="multiprocessing.connection.Connection.recv"><code class="xref py py-meth docutils literal notranslate"><span class="pre">recv()</span></code></a>.</p>

  1402. <p>The object must be picklable.  Very large pickles (approximately 32 MiB+,

  1403. though it depends on the OS) may raise a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> exception.</p>

  1404. </dd></dl>

  1405. 

  1406. <dl class="py method">

  1407. <dt class="sig sig-object py" id="multiprocessing.connection.Connection.recv">

  1408. <span class="sig-name descname"><span class="pre">recv</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Connection.recv" title="Link to this definition">¶</a></dt>

  1409. <dd><p>Return an object sent from the other end of the connection using

  1410. <a class="reference internal" href="#multiprocessing.connection.Connection.send" title="multiprocessing.connection.Connection.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a>.  Blocks until there is something to receive.  Raises

  1411. <a class="reference internal" href="exceptions.html#EOFError" title="EOFError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">EOFError</span></code></a> if there is nothing left to receive

  1412. and the other end was closed.</p>

  1413. </dd></dl>

  1414. 

  1415. <dl class="py method">

  1416. <dt class="sig sig-object py" id="multiprocessing.connection.Connection.fileno">

  1417. <span class="sig-name descname"><span class="pre">fileno</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Connection.fileno" title="Link to this definition">¶</a></dt>

  1418. <dd><p>Return the file descriptor or handle used by the connection.</p>

  1419. </dd></dl>

  1420. 

  1421. <dl class="py method">

  1422. <dt class="sig sig-object py" id="multiprocessing.connection.Connection.close">

  1423. <span class="sig-name descname"><span class="pre">close</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Connection.close" title="Link to this definition">¶</a></dt>

  1424. <dd><p>Close the connection.</p>

  1425. <p>This is called automatically when the connection is garbage collected.</p>

  1426. </dd></dl>

  1427. 

  1428. <dl class="py method">

  1429. <dt class="sig sig-object py" id="multiprocessing.connection.Connection.poll">

  1430. <span class="sig-name descname"><span class="pre">poll</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">timeout</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Connection.poll" title="Link to this definition">¶</a></dt>

  1431. <dd><p>Return whether there is any data available to be read.</p>

  1432. <p>If <em>timeout</em> is not specified then it will return immediately.  If

  1433. <em>timeout</em> is a number then this specifies the maximum time in seconds to

  1434. block.  If <em>timeout</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> then an infinite timeout is used.</p>

  1435. <p>Note that multiple connection objects may be polled at once by

  1436. using <a class="reference internal" href="#multiprocessing.connection.wait" title="multiprocessing.connection.wait"><code class="xref py py-func docutils literal notranslate"><span class="pre">multiprocessing.connection.wait()</span></code></a>.</p>

  1437. </dd></dl>

  1438. 

  1439. <dl class="py method">

  1440. <dt class="sig sig-object py" id="multiprocessing.connection.Connection.send_bytes">

  1441. <span class="sig-name descname"><span class="pre">send_bytes</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">offset</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">size</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Connection.send_bytes" title="Link to this definition">¶</a></dt>

  1442. <dd><p>Send byte data from a <a class="reference internal" href="../glossary.html#term-bytes-like-object"><span class="xref std std-term">bytes-like object</span></a> as a complete message.</p>

  1443. <p>If <em>offset</em> is given then data is read from that position in <em>buffer</em>.  If

  1444. <em>size</em> is given then that many bytes will be read from buffer.  Very large

  1445. buffers (approximately 32 MiB+, though it depends on the OS) may raise a

  1446. <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> exception</p>

  1447. </dd></dl>

  1448. 

  1449. <dl class="py method">

  1450. <dt class="sig sig-object py" id="multiprocessing.connection.Connection.recv_bytes">

  1451. <span class="sig-name descname"><span class="pre">recv_bytes</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">maxlength</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Connection.recv_bytes" title="Link to this definition">¶</a></dt>

  1452. <dd><p>Return a complete message of byte data sent from the other end of the

  1453. connection as a string.  Blocks until there is something to receive.

  1454. Raises <a class="reference internal" href="exceptions.html#EOFError" title="EOFError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">EOFError</span></code></a> if there is nothing left

  1455. to receive and the other end has closed.</p>

  1456. <p>If <em>maxlength</em> is specified and the message is longer than <em>maxlength</em>

  1457. then <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a> is raised and the connection will no longer be

  1458. readable.</p>

  1459. <div class="versionchanged">

  1460. <p><span class="versionmodified changed">Changed in version 3.3: </span>This function used to raise <a class="reference internal" href="exceptions.html#IOError" title="IOError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IOError</span></code></a>, which is now an

  1461. alias of <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a>.</p>

  1462. </div>

  1463. </dd></dl>

  1464. 

  1465. <dl class="py method">

  1466. <dt class="sig sig-object py" id="multiprocessing.connection.Connection.recv_bytes_into">

  1467. <span class="sig-name descname"><span class="pre">recv_bytes_into</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">offset</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Connection.recv_bytes_into" title="Link to this definition">¶</a></dt>

  1468. <dd><p>Read into <em>buffer</em> a complete message of byte data sent from the other end

  1469. of the connection and return the number of bytes in the message.  Blocks

  1470. until there is something to receive.  Raises

  1471. <a class="reference internal" href="exceptions.html#EOFError" title="EOFError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">EOFError</span></code></a> if there is nothing left to receive and the other end was

  1472. closed.</p>

  1473. <p><em>buffer</em> must be a writable <a class="reference internal" href="../glossary.html#term-bytes-like-object"><span class="xref std std-term">bytes-like object</span></a>.  If

  1474. <em>offset</em> is given then the message will be written into the buffer from

  1475. that position.  Offset must be a non-negative integer less than the

  1476. length of <em>buffer</em> (in bytes).</p>

  1477. <p>If the buffer is too short then a <code class="xref py py-exc docutils literal notranslate"><span class="pre">BufferTooShort</span></code> exception is

  1478. raised and the complete message is available as <code class="docutils literal notranslate"><span class="pre">e.args[0]</span></code> where <code class="docutils literal notranslate"><span class="pre">e</span></code>

  1479. is the exception instance.</p>

  1480. </dd></dl>

  1481. 

  1482. <div class="versionchanged">

  1483. <p><span class="versionmodified changed">Changed in version 3.3: </span>Connection objects themselves can now be transferred between processes

  1484. using <a class="reference internal" href="#multiprocessing.connection.Connection.send" title="multiprocessing.connection.Connection.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Connection.send()</span></code></a> and <a class="reference internal" href="#multiprocessing.connection.Connection.recv" title="multiprocessing.connection.Connection.recv"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Connection.recv()</span></code></a>.</p>

  1485. <p>Connection objects also now support the context management protocol – see

  1486. <a class="reference internal" href="stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a>.  <a class="reference internal" href="stdtypes.html#contextmanager.__enter__" title="contextmanager.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> returns the

  1487. connection object, and <a class="reference internal" href="stdtypes.html#contextmanager.__exit__" title="contextmanager.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> calls <a class="reference internal" href="#multiprocessing.connection.Connection.close" title="multiprocessing.connection.Connection.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a>.</p>

  1488. </div>

  1489. </dd></dl>

  1490. 

  1491. <p>For example:</p>

  1492. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Pipe</span>

  1493. <span class="gp">&gt;&gt;&gt; </span><span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">Pipe</span><span class="p">()</span>

  1494. <span class="gp">&gt;&gt;&gt; </span><span class="n">a</span><span class="o">.</span><span class="n">send</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="s1">&#39;hello&#39;</span><span class="p">,</span> <span class="kc">None</span><span class="p">])</span>

  1495. <span class="gp">&gt;&gt;&gt; </span><span class="n">b</span><span class="o">.</span><span class="n">recv</span><span class="p">()</span>

  1496. <span class="go">[1, &#39;hello&#39;, None]</span>

  1497. <span class="gp">&gt;&gt;&gt; </span><span class="n">b</span><span class="o">.</span><span class="n">send_bytes</span><span class="p">(</span><span class="sa">b</span><span class="s1">&#39;thank you&#39;</span><span class="p">)</span>

  1498. <span class="gp">&gt;&gt;&gt; </span><span class="n">a</span><span class="o">.</span><span class="n">recv_bytes</span><span class="p">()</span>

  1499. <span class="go">b&#39;thank you&#39;</span>

  1500. <span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">array</span>

  1501. <span class="gp">&gt;&gt;&gt; </span><span class="n">arr1</span> <span class="o">=</span> <span class="n">array</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="s1">&#39;i&#39;</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">))</span>

  1502. <span class="gp">&gt;&gt;&gt; </span><span class="n">arr2</span> <span class="o">=</span> <span class="n">array</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="s1">&#39;i&#39;</span><span class="p">,</span> <span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">*</span> <span class="mi">10</span><span class="p">)</span>

  1503. <span class="gp">&gt;&gt;&gt; </span><span class="n">a</span><span class="o">.</span><span class="n">send_bytes</span><span class="p">(</span><span class="n">arr1</span><span class="p">)</span>

  1504. <span class="gp">&gt;&gt;&gt; </span><span class="n">count</span> <span class="o">=</span> <span class="n">b</span><span class="o">.</span><span class="n">recv_bytes_into</span><span class="p">(</span><span class="n">arr2</span><span class="p">)</span>

  1505. <span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="n">count</span> <span class="o">==</span> <span class="nb">len</span><span class="p">(</span><span class="n">arr1</span><span class="p">)</span> <span class="o">*</span> <span class="n">arr1</span><span class="o">.</span><span class="n">itemsize</span>

  1506. <span class="gp">&gt;&gt;&gt; </span><span class="n">arr2</span>

  1507. <span class="go">array(&#39;i&#39;, [0, 1, 2, 3, 4, 0, 0, 0, 0, 0])</span>

  1508. </pre></div>

  1509. </div>

  1510. <div class="admonition warning" id="multiprocessing-recv-pickle-security">

  1511. <p class="admonition-title">Warning</p>

  1512. <p>The <a class="reference internal" href="#multiprocessing.connection.Connection.recv" title="multiprocessing.connection.Connection.recv"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Connection.recv()</span></code></a> method automatically unpickles the data it

  1513. receives, which can be a security risk unless you can trust the process

  1514. which sent the message.</p>

  1515. <p>Therefore, unless the connection object was produced using <code class="xref py py-func docutils literal notranslate"><span class="pre">Pipe()</span></code> you

  1516. should only use the <a class="reference internal" href="#multiprocessing.connection.Connection.recv" title="multiprocessing.connection.Connection.recv"><code class="xref py py-meth docutils literal notranslate"><span class="pre">recv()</span></code></a> and <a class="reference internal" href="#multiprocessing.connection.Connection.send" title="multiprocessing.connection.Connection.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a>

  1517. methods after performing some sort of authentication.  See

  1518. <a class="reference internal" href="#multiprocessing-auth-keys"><span class="std std-ref">Authentication keys</span></a>.</p>

  1519. </div>

  1520. <div class="admonition warning">

  1521. <p class="admonition-title">Warning</p>

  1522. <p>If a process is killed while it is trying to read or write to a pipe then

  1523. the data in the pipe is likely to become corrupted, because it may become

  1524. impossible to be sure where the message boundaries lie.</p>

  1525. </div>

  1526. </section>

  1527. <section id="synchronization-primitives">

  1528. <h3>Synchronization primitives<a class="headerlink" href="#synchronization-primitives" title="Link to this heading">¶</a></h3>

  1529. <p>Generally synchronization primitives are not as necessary in a multiprocess

  1530. program as they are in a multithreaded program.  See the documentation for

  1531. <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module.</p>

  1532. <p>Note that one can also create synchronization primitives by using a manager

  1533. object – see <a class="reference internal" href="#multiprocessing-managers"><span class="std std-ref">Managers</span></a>.</p>

  1534. <dl class="py class">

  1535. <dt class="sig sig-object py" id="multiprocessing.Barrier">

  1536. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Barrier</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">parties</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">action</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Barrier" title="Link to this definition">¶</a></dt>

  1537. <dd><p>A barrier object: a clone of <a class="reference internal" href="threading.html#threading.Barrier" title="threading.Barrier"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Barrier</span></code></a>.</p>

  1538. <div class="versionadded">

  1539. <p><span class="versionmodified added">New in version 3.3.</span></p>

  1540. </div>

  1541. </dd></dl>

  1542. 

  1543. <dl class="py class">

  1544. <dt class="sig sig-object py" id="multiprocessing.BoundedSemaphore">

  1545. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">BoundedSemaphore</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.BoundedSemaphore" title="Link to this definition">¶</a></dt>

  1546. <dd><p>A bounded semaphore object: a close analog of

  1547. <a class="reference internal" href="threading.html#threading.BoundedSemaphore" title="threading.BoundedSemaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.BoundedSemaphore</span></code></a>.</p>

  1548. <p>A solitary difference from its close analog exists: its <code class="docutils literal notranslate"><span class="pre">acquire</span></code> method’s

  1549. first argument is named <em>block</em>, as is consistent with <a class="reference internal" href="#multiprocessing.Lock.acquire" title="multiprocessing.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Lock.acquire()</span></code></a>.</p>

  1550. <div class="admonition note">

  1551. <p class="admonition-title">Note</p>

  1552. <p>On macOS, this is indistinguishable from <a class="reference internal" href="#multiprocessing.Semaphore" title="multiprocessing.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">Semaphore</span></code></a> because

  1553. <code class="docutils literal notranslate"><span class="pre">sem_getvalue()</span></code> is not implemented on that platform.</p>

  1554. </div>

  1555. </dd></dl>

  1556. 

  1557. <dl class="py class">

  1558. <dt class="sig sig-object py" id="multiprocessing.Condition">

  1559. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Condition</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">lock</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Condition" title="Link to this definition">¶</a></dt>

  1560. <dd><p>A condition variable: an alias for <a class="reference internal" href="threading.html#threading.Condition" title="threading.Condition"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Condition</span></code></a>.</p>

  1561. <p>If <em>lock</em> is specified then it should be a <a class="reference internal" href="#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> or <a class="reference internal" href="#multiprocessing.RLock" title="multiprocessing.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a>

  1562. object from <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a>.</p>

  1563. <div class="versionchanged">

  1564. <p><span class="versionmodified changed">Changed in version 3.3: </span>The <a class="reference internal" href="threading.html#threading.Condition.wait_for" title="threading.Condition.wait_for"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait_for()</span></code></a> method was added.</p>

  1565. </div>

  1566. </dd></dl>

  1567. 

  1568. <dl class="py class">

  1569. <dt class="sig sig-object py" id="multiprocessing.Event">

  1570. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Event</span></span><a class="headerlink" href="#multiprocessing.Event" title="Link to this definition">¶</a></dt>

  1571. <dd><p>A clone of <a class="reference internal" href="threading.html#threading.Event" title="threading.Event"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Event</span></code></a>.</p>

  1572. </dd></dl>

  1573. 

  1574. <dl class="py class">

  1575. <dt class="sig sig-object py" id="multiprocessing.Lock">

  1576. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Lock</span></span><a class="headerlink" href="#multiprocessing.Lock" title="Link to this definition">¶</a></dt>

  1577. <dd><p>A non-recursive lock object: a close analog of <a class="reference internal" href="threading.html#threading.Lock" title="threading.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Lock</span></code></a>.

  1578. Once a process or thread has acquired a lock, subsequent attempts to

  1579. acquire it from any process or thread will block until it is released;

  1580. any process or thread may release it.  The concepts and behaviors of

  1581. <a class="reference internal" href="threading.html#threading.Lock" title="threading.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Lock</span></code></a> as it applies to threads are replicated here in

  1582. <a class="reference internal" href="#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">multiprocessing.Lock</span></code></a> as it applies to either processes or threads,

  1583. except as noted.</p>

  1584. <p>Note that <a class="reference internal" href="#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> is actually a factory function which returns an

  1585. instance of <code class="docutils literal notranslate"><span class="pre">multiprocessing.synchronize.Lock</span></code> initialized with a

  1586. default context.</p>

  1587. <p><a class="reference internal" href="#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> supports the <a class="reference internal" href="../glossary.html#term-context-manager"><span class="xref std std-term">context manager</span></a> protocol and thus may be

  1588. used in <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statements.</p>

  1589. <dl class="py method">

  1590. <dt class="sig sig-object py" id="multiprocessing.Lock.acquire">

  1591. <span class="sig-name descname"><span class="pre">acquire</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">block</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">True</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Lock.acquire" title="Link to this definition">¶</a></dt>

  1592. <dd><p>Acquire a lock, blocking or non-blocking.</p>

  1593. <p>With the <em>block</em> argument set to <code class="docutils literal notranslate"><span class="pre">True</span></code> (the default), the method call

  1594. will block until the lock is in an unlocked state, then set it to locked

  1595. and return <code class="docutils literal notranslate"><span class="pre">True</span></code>.  Note that the name of this first argument differs

  1596. from that in <a class="reference internal" href="threading.html#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">threading.Lock.acquire()</span></code></a>.</p>

  1597. <p>With the <em>block</em> argument set to <code class="docutils literal notranslate"><span class="pre">False</span></code>, the method call does not

  1598. block.  If the lock is currently in a locked state, return <code class="docutils literal notranslate"><span class="pre">False</span></code>;

  1599. otherwise set the lock to a locked state and return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  1600. <p>When invoked with a positive, floating-point value for <em>timeout</em>, block

  1601. for at most the number of seconds specified by <em>timeout</em> as long as

  1602. the lock can not be acquired.  Invocations with a negative value for

  1603. <em>timeout</em> are equivalent to a <em>timeout</em> of zero.  Invocations with a

  1604. <em>timeout</em> value of <code class="docutils literal notranslate"><span class="pre">None</span></code> (the default) set the timeout period to

  1605. infinite.  Note that the treatment of negative or <code class="docutils literal notranslate"><span class="pre">None</span></code> values for

  1606. <em>timeout</em> differs from the implemented behavior in

  1607. <a class="reference internal" href="threading.html#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">threading.Lock.acquire()</span></code></a>.  The <em>timeout</em> argument has no practical

  1608. implications if the <em>block</em> argument is set to <code class="docutils literal notranslate"><span class="pre">False</span></code> and is thus

  1609. ignored.  Returns <code class="docutils literal notranslate"><span class="pre">True</span></code> if the lock has been acquired or <code class="docutils literal notranslate"><span class="pre">False</span></code> if

  1610. the timeout period has elapsed.</p>

  1611. </dd></dl>

  1612. 

  1613. <dl class="py method">

  1614. <dt class="sig sig-object py" id="multiprocessing.Lock.release">

  1615. <span class="sig-name descname"><span class="pre">release</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Lock.release" title="Link to this definition">¶</a></dt>

  1616. <dd><p>Release a lock.  This can be called from any process or thread, not only

  1617. the process or thread which originally acquired the lock.</p>

  1618. <p>Behavior is the same as in <a class="reference internal" href="threading.html#threading.Lock.release" title="threading.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">threading.Lock.release()</span></code></a> except that

  1619. when invoked on an unlocked lock, a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised.</p>

  1620. </dd></dl>

  1621. 

  1622. </dd></dl>

  1623. 

  1624. <dl class="py class">

  1625. <dt class="sig sig-object py" id="multiprocessing.RLock">

  1626. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">RLock</span></span><a class="headerlink" href="#multiprocessing.RLock" title="Link to this definition">¶</a></dt>

  1627. <dd><p>A recursive lock object: a close analog of <a class="reference internal" href="threading.html#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.RLock</span></code></a>.  A

  1628. recursive lock must be released by the process or thread that acquired it.

  1629. Once a process or thread has acquired a recursive lock, the same process

  1630. or thread may acquire it again without blocking; that process or thread

  1631. must release it once for each time it has been acquired.</p>

  1632. <p>Note that <a class="reference internal" href="#multiprocessing.RLock" title="multiprocessing.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a> is actually a factory function which returns an

  1633. instance of <code class="docutils literal notranslate"><span class="pre">multiprocessing.synchronize.RLock</span></code> initialized with a

  1634. default context.</p>

  1635. <p><a class="reference internal" href="#multiprocessing.RLock" title="multiprocessing.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a> supports the <a class="reference internal" href="../glossary.html#term-context-manager"><span class="xref std std-term">context manager</span></a> protocol and thus may be

  1636. used in <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statements.</p>

  1637. <dl class="py method">

  1638. <dt class="sig sig-object py" id="multiprocessing.RLock.acquire">

  1639. <span class="sig-name descname"><span class="pre">acquire</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">block</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">True</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.RLock.acquire" title="Link to this definition">¶</a></dt>

  1640. <dd><p>Acquire a lock, blocking or non-blocking.</p>

  1641. <p>When invoked with the <em>block</em> argument set to <code class="docutils literal notranslate"><span class="pre">True</span></code>, block until the

  1642. lock is in an unlocked state (not owned by any process or thread) unless

  1643. the lock is already owned by the current process or thread.  The current

  1644. process or thread then takes ownership of the lock (if it does not

  1645. already have ownership) and the recursion level inside the lock increments

  1646. by one, resulting in a return value of <code class="docutils literal notranslate"><span class="pre">True</span></code>.  Note that there are

  1647. several differences in this first argument’s behavior compared to the

  1648. implementation of <a class="reference internal" href="threading.html#threading.RLock.acquire" title="threading.RLock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">threading.RLock.acquire()</span></code></a>, starting with the name

  1649. of the argument itself.</p>

  1650. <p>When invoked with the <em>block</em> argument set to <code class="docutils literal notranslate"><span class="pre">False</span></code>, do not block.

  1651. If the lock has already been acquired (and thus is owned) by another

  1652. process or thread, the current process or thread does not take ownership

  1653. and the recursion level within the lock is not changed, resulting in

  1654. a return value of <code class="docutils literal notranslate"><span class="pre">False</span></code>.  If the lock is in an unlocked state, the

  1655. current process or thread takes ownership and the recursion level is

  1656. incremented, resulting in a return value of <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  1657. <p>Use and behaviors of the <em>timeout</em> argument are the same as in

  1658. <a class="reference internal" href="#multiprocessing.Lock.acquire" title="multiprocessing.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Lock.acquire()</span></code></a>.  Note that some of these behaviors of <em>timeout</em>

  1659. differ from the implemented behaviors in <a class="reference internal" href="threading.html#threading.RLock.acquire" title="threading.RLock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">threading.RLock.acquire()</span></code></a>.</p>

  1660. </dd></dl>

  1661. 

  1662. <dl class="py method">

  1663. <dt class="sig sig-object py" id="multiprocessing.RLock.release">

  1664. <span class="sig-name descname"><span class="pre">release</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.RLock.release" title="Link to this definition">¶</a></dt>

  1665. <dd><p>Release a lock, decrementing the recursion level.  If after the

  1666. decrement the recursion level is zero, reset the lock to unlocked (not

  1667. owned by any process or thread) and if any other processes or threads

  1668. are blocked waiting for the lock to become unlocked, allow exactly one

  1669. of them to proceed.  If after the decrement the recursion level is still

  1670. nonzero, the lock remains locked and owned by the calling process or

  1671. thread.</p>

  1672. <p>Only call this method when the calling process or thread owns the lock.

  1673. An <a class="reference internal" href="exceptions.html#AssertionError" title="AssertionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AssertionError</span></code></a> is raised if this method is called by a process

  1674. or thread other than the owner or if the lock is in an unlocked (unowned)

  1675. state.  Note that the type of exception raised in this situation

  1676. differs from the implemented behavior in <a class="reference internal" href="threading.html#threading.RLock.release" title="threading.RLock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">threading.RLock.release()</span></code></a>.</p>

  1677. </dd></dl>

  1678. 

  1679. </dd></dl>

  1680. 

  1681. <dl class="py class">

  1682. <dt class="sig sig-object py" id="multiprocessing.Semaphore">

  1683. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Semaphore</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Semaphore" title="Link to this definition">¶</a></dt>

  1684. <dd><p>A semaphore object: a close analog of <a class="reference internal" href="threading.html#threading.Semaphore" title="threading.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Semaphore</span></code></a>.</p>

  1685. <p>A solitary difference from its close analog exists: its <code class="docutils literal notranslate"><span class="pre">acquire</span></code> method’s

  1686. first argument is named <em>block</em>, as is consistent with <a class="reference internal" href="#multiprocessing.Lock.acquire" title="multiprocessing.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Lock.acquire()</span></code></a>.</p>

  1687. </dd></dl>

  1688. 

  1689. <div class="admonition note">

  1690. <p class="admonition-title">Note</p>

  1691. <p>On macOS, <code class="docutils literal notranslate"><span class="pre">sem_timedwait</span></code> is unsupported, so calling <code class="docutils literal notranslate"><span class="pre">acquire()</span></code> with

  1692. a timeout will emulate that function’s behavior using a sleeping loop.</p>

  1693. </div>

  1694. <div class="admonition note">

  1695. <p class="admonition-title">Note</p>

  1696. <p>If the SIGINT signal generated by <kbd class="kbd compound docutils literal notranslate"><kbd class="kbd docutils literal notranslate">Ctrl</kbd>-<kbd class="kbd docutils literal notranslate">C</kbd></kbd> arrives while the main thread is

  1697. blocked by a call to <code class="xref py py-meth docutils literal notranslate"><span class="pre">BoundedSemaphore.acquire()</span></code>, <a class="reference internal" href="#multiprocessing.Lock.acquire" title="multiprocessing.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Lock.acquire()</span></code></a>,

  1698. <a class="reference internal" href="#multiprocessing.RLock.acquire" title="multiprocessing.RLock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">RLock.acquire()</span></code></a>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">Semaphore.acquire()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">Condition.acquire()</span></code>

  1699. or <code class="xref py py-meth docutils literal notranslate"><span class="pre">Condition.wait()</span></code> then the call will be immediately interrupted and

  1700. <a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> will be raised.</p>

  1701. <p>This differs from the behaviour of <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> where SIGINT will be

  1702. ignored while the equivalent blocking calls are in progress.</p>

  1703. </div>

  1704. <div class="admonition note">

  1705. <p class="admonition-title">Note</p>

  1706. <p>Some of this package’s functionality requires a functioning shared semaphore

  1707. implementation on the host operating system. Without one, the

  1708. <code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.synchronize</span></code> module will be disabled, and attempts to

  1709. import it will result in an <a class="reference internal" href="exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a>. See

  1710. <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=3770">bpo-3770</a> for additional information.</p>

  1711. </div>

  1712. </section>

  1713. <section id="shared-ctypes-objects">

  1714. <h3>Shared <a class="reference internal" href="ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> Objects<a class="headerlink" href="#shared-ctypes-objects" title="Link to this heading">¶</a></h3>

  1715. <p>It is possible to create shared objects using shared memory which can be

  1716. inherited by child processes.</p>

  1717. <dl class="py function">

  1718. <dt class="sig sig-object py" id="multiprocessing.Value">

  1719. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Value</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">typecode_or_type</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span><span class="n"><span class="pre">args</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">lock</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">True</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Value" title="Link to this definition">¶</a></dt>

  1720. <dd><p>Return a <a class="reference internal" href="ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> object allocated from shared memory.  By default the

  1721. return value is actually a synchronized wrapper for the object.  The object

  1722. itself can be accessed via the <em>value</em> attribute of a <a class="reference internal" href="#multiprocessing.Value" title="multiprocessing.Value"><code class="xref py py-class docutils literal notranslate"><span class="pre">Value</span></code></a>.</p>

  1723. <p><em>typecode_or_type</em> determines the type of the returned object: it is either a

  1724. ctypes type or a one character typecode of the kind used by the <a class="reference internal" href="array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><code class="xref py py-mod docutils literal notranslate"><span class="pre">array</span></code></a>

  1725. module.  <em>*args</em> is passed on to the constructor for the type.</p>

  1726. <p>If <em>lock</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code> (the default) then a new recursive lock

  1727. object is created to synchronize access to the value.  If <em>lock</em> is

  1728. a <a class="reference internal" href="#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> or <a class="reference internal" href="#multiprocessing.RLock" title="multiprocessing.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a> object then that will be used to

  1729. synchronize access to the value.  If <em>lock</em> is <code class="docutils literal notranslate"><span class="pre">False</span></code> then

  1730. access to the returned object will not be automatically protected

  1731. by a lock, so it will not necessarily be “process-safe”.</p>

  1732. <p>Operations like <code class="docutils literal notranslate"><span class="pre">+=</span></code> which involve a read and write are not

  1733. atomic.  So if, for instance, you want to atomically increment a

  1734. shared value it is insufficient to just do</p>

  1735. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">counter</span><span class="o">.</span><span class="n">value</span> <span class="o">+=</span> <span class="mi">1</span>

  1736. </pre></div>

  1737. </div>

  1738. <p>Assuming the associated lock is recursive (which it is by default)

  1739. you can instead do</p>

  1740. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">counter</span><span class="o">.</span><span class="n">get_lock</span><span class="p">():</span>

  1741.     <span class="n">counter</span><span class="o">.</span><span class="n">value</span> <span class="o">+=</span> <span class="mi">1</span>

  1742. </pre></div>

  1743. </div>

  1744. <p>Note that <em>lock</em> is a keyword-only argument.</p>

  1745. </dd></dl>

  1746. 

  1747. <dl class="py function">

  1748. <dt class="sig sig-object py" id="multiprocessing.Array">

  1749. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Array</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">typecode_or_type</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">size_or_initializer</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">lock</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">True</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Array" title="Link to this definition">¶</a></dt>

  1750. <dd><p>Return a ctypes array allocated from shared memory.  By default the return

  1751. value is actually a synchronized wrapper for the array.</p>

  1752. <p><em>typecode_or_type</em> determines the type of the elements of the returned array:

  1753. it is either a ctypes type or a one character typecode of the kind used by

  1754. the <a class="reference internal" href="array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><code class="xref py py-mod docutils literal notranslate"><span class="pre">array</span></code></a> module.  If <em>size_or_initializer</em> is an integer, then it

  1755. determines the length of the array, and the array will be initially zeroed.

  1756. Otherwise, <em>size_or_initializer</em> is a sequence which is used to initialize

  1757. the array and whose length determines the length of the array.</p>

  1758. <p>If <em>lock</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code> (the default) then a new lock object is created to

  1759. synchronize access to the value.  If <em>lock</em> is a <a class="reference internal" href="#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> or

  1760. <a class="reference internal" href="#multiprocessing.RLock" title="multiprocessing.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a> object then that will be used to synchronize access to the

  1761. value.  If <em>lock</em> is <code class="docutils literal notranslate"><span class="pre">False</span></code> then access to the returned object will not be

  1762. automatically protected by a lock, so it will not necessarily be

  1763. “process-safe”.</p>

  1764. <p>Note that <em>lock</em> is a keyword only argument.</p>

  1765. <p>Note that an array of <a class="reference internal" href="ctypes.html#ctypes.c_char" title="ctypes.c_char"><code class="xref py py-data docutils literal notranslate"><span class="pre">ctypes.c_char</span></code></a> has <em>value</em> and <em>raw</em>

  1766. attributes which allow one to use it to store and retrieve strings.</p>

  1767. </dd></dl>

  1768. 

  1769. <section id="module-multiprocessing.sharedctypes">

  1770. <span id="the-multiprocessing-sharedctypes-module"></span><h4>The <a class="reference internal" href="#module-multiprocessing.sharedctypes" title="multiprocessing.sharedctypes: Allocate ctypes objects from shared memory."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.sharedctypes</span></code></a> module<a class="headerlink" href="#module-multiprocessing.sharedctypes" title="Link to this heading">¶</a></h4>

  1771. <p>The <a class="reference internal" href="#module-multiprocessing.sharedctypes" title="multiprocessing.sharedctypes: Allocate ctypes objects from shared memory."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.sharedctypes</span></code></a> module provides functions for allocating

  1772. <a class="reference internal" href="ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> objects from shared memory which can be inherited by child

  1773. processes.</p>

  1774. <div class="admonition note">

  1775. <p class="admonition-title">Note</p>

  1776. <p>Although it is possible to store a pointer in shared memory remember that

  1777. this will refer to a location in the address space of a specific process.

  1778. However, the pointer is quite likely to be invalid in the context of a second

  1779. process and trying to dereference the pointer from the second process may

  1780. cause a crash.</p>

  1781. </div>

  1782. <dl class="py function">

  1783. <dt class="sig sig-object py" id="multiprocessing.sharedctypes.RawArray">

  1784. <span class="sig-prename descclassname"><span class="pre">multiprocessing.sharedctypes.</span></span><span class="sig-name descname"><span class="pre">RawArray</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">typecode_or_type</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">size_or_initializer</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.sharedctypes.RawArray" title="Link to this definition">¶</a></dt>

  1785. <dd><p>Return a ctypes array allocated from shared memory.</p>

  1786. <p><em>typecode_or_type</em> determines the type of the elements of the returned array:

  1787. it is either a ctypes type or a one character typecode of the kind used by

  1788. the <a class="reference internal" href="array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><code class="xref py py-mod docutils literal notranslate"><span class="pre">array</span></code></a> module.  If <em>size_or_initializer</em> is an integer then it

  1789. determines the length of the array, and the array will be initially zeroed.

  1790. Otherwise <em>size_or_initializer</em> is a sequence which is used to initialize the

  1791. array and whose length determines the length of the array.</p>

  1792. <p>Note that setting and getting an element is potentially non-atomic – use

  1793. <a class="reference internal" href="#multiprocessing.sharedctypes.Array" title="multiprocessing.sharedctypes.Array"><code class="xref py py-func docutils literal notranslate"><span class="pre">Array()</span></code></a> instead to make sure that access is automatically synchronized

  1794. using a lock.</p>

  1795. </dd></dl>

  1796. 

  1797. <dl class="py function">

  1798. <dt class="sig sig-object py" id="multiprocessing.sharedctypes.RawValue">

  1799. <span class="sig-prename descclassname"><span class="pre">multiprocessing.sharedctypes.</span></span><span class="sig-name descname"><span class="pre">RawValue</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">typecode_or_type</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span><span class="n"><span class="pre">args</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.sharedctypes.RawValue" title="Link to this definition">¶</a></dt>

  1800. <dd><p>Return a ctypes object allocated from shared memory.</p>

  1801. <p><em>typecode_or_type</em> determines the type of the returned object: it is either a

  1802. ctypes type or a one character typecode of the kind used by the <a class="reference internal" href="array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><code class="xref py py-mod docutils literal notranslate"><span class="pre">array</span></code></a>

  1803. module.  <em>*args</em> is passed on to the constructor for the type.</p>

  1804. <p>Note that setting and getting the value is potentially non-atomic – use

  1805. <a class="reference internal" href="#multiprocessing.sharedctypes.Value" title="multiprocessing.sharedctypes.Value"><code class="xref py py-func docutils literal notranslate"><span class="pre">Value()</span></code></a> instead to make sure that access is automatically synchronized

  1806. using a lock.</p>

  1807. <p>Note that an array of <a class="reference internal" href="ctypes.html#ctypes.c_char" title="ctypes.c_char"><code class="xref py py-data docutils literal notranslate"><span class="pre">ctypes.c_char</span></code></a> has <code class="docutils literal notranslate"><span class="pre">value</span></code> and <code class="docutils literal notranslate"><span class="pre">raw</span></code>

  1808. attributes which allow one to use it to store and retrieve strings – see

  1809. documentation for <a class="reference internal" href="ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a>.</p>

  1810. </dd></dl>

  1811. 

  1812. <dl class="py function">

  1813. <dt class="sig sig-object py" id="multiprocessing.sharedctypes.Array">

  1814. <span class="sig-prename descclassname"><span class="pre">multiprocessing.sharedctypes.</span></span><span class="sig-name descname"><span class="pre">Array</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">typecode_or_type</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">size_or_initializer</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">lock</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">True</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.sharedctypes.Array" title="Link to this definition">¶</a></dt>

  1815. <dd><p>The same as <a class="reference internal" href="#multiprocessing.sharedctypes.RawArray" title="multiprocessing.sharedctypes.RawArray"><code class="xref py py-func docutils literal notranslate"><span class="pre">RawArray()</span></code></a> except that depending on the value of <em>lock</em> a

  1816. process-safe synchronization wrapper may be returned instead of a raw ctypes

  1817. array.</p>

  1818. <p>If <em>lock</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code> (the default) then a new lock object is created to

  1819. synchronize access to the value.  If <em>lock</em> is a

  1820. <a class="reference internal" href="#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> or <a class="reference internal" href="#multiprocessing.RLock" title="multiprocessing.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a> object

  1821. then that will be used to synchronize access to the

  1822. value.  If <em>lock</em> is <code class="docutils literal notranslate"><span class="pre">False</span></code> then access to the returned object will not be

  1823. automatically protected by a lock, so it will not necessarily be

  1824. “process-safe”.</p>

  1825. <p>Note that <em>lock</em> is a keyword-only argument.</p>

  1826. </dd></dl>

  1827. 

  1828. <dl class="py function">

  1829. <dt class="sig sig-object py" id="multiprocessing.sharedctypes.Value">

  1830. <span class="sig-prename descclassname"><span class="pre">multiprocessing.sharedctypes.</span></span><span class="sig-name descname"><span class="pre">Value</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">typecode_or_type</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span><span class="n"><span class="pre">args</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">lock</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">True</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.sharedctypes.Value" title="Link to this definition">¶</a></dt>

  1831. <dd><p>The same as <a class="reference internal" href="#multiprocessing.sharedctypes.RawValue" title="multiprocessing.sharedctypes.RawValue"><code class="xref py py-func docutils literal notranslate"><span class="pre">RawValue()</span></code></a> except that depending on the value of <em>lock</em> a

  1832. process-safe synchronization wrapper may be returned instead of a raw ctypes

  1833. object.</p>

  1834. <p>If <em>lock</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code> (the default) then a new lock object is created to

  1835. synchronize access to the value.  If <em>lock</em> is a <a class="reference internal" href="#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> or

  1836. <a class="reference internal" href="#multiprocessing.RLock" title="multiprocessing.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a> object then that will be used to synchronize access to the

  1837. value.  If <em>lock</em> is <code class="docutils literal notranslate"><span class="pre">False</span></code> then access to the returned object will not be

  1838. automatically protected by a lock, so it will not necessarily be

  1839. “process-safe”.</p>

  1840. <p>Note that <em>lock</em> is a keyword-only argument.</p>

  1841. </dd></dl>

  1842. 

  1843. <dl class="py function">

  1844. <dt class="sig sig-object py" id="multiprocessing.sharedctypes.copy">

  1845. <span class="sig-prename descclassname"><span class="pre">multiprocessing.sharedctypes.</span></span><span class="sig-name descname"><span class="pre">copy</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">obj</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.sharedctypes.copy" title="Link to this definition">¶</a></dt>

  1846. <dd><p>Return a ctypes object allocated from shared memory which is a copy of the

  1847. ctypes object <em>obj</em>.</p>

  1848. </dd></dl>

  1849. 

  1850. <dl class="py function">

  1851. <dt class="sig sig-object py" id="multiprocessing.sharedctypes.synchronized">

  1852. <span class="sig-prename descclassname"><span class="pre">multiprocessing.sharedctypes.</span></span><span class="sig-name descname"><span class="pre">synchronized</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">obj</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">lock</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.sharedctypes.synchronized" title="Link to this definition">¶</a></dt>

  1853. <dd><p>Return a process-safe wrapper object for a ctypes object which uses <em>lock</em> to

  1854. synchronize access.  If <em>lock</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> (the default) then a

  1855. <a class="reference internal" href="#multiprocessing.RLock" title="multiprocessing.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">multiprocessing.RLock</span></code></a> object is created automatically.</p>

  1856. <p>A synchronized wrapper will have two methods in addition to those of the

  1857. object it wraps: <code class="xref py py-meth docutils literal notranslate"><span class="pre">get_obj()</span></code> returns the wrapped object and

  1858. <code class="xref py py-meth docutils literal notranslate"><span class="pre">get_lock()</span></code> returns the lock object used for synchronization.</p>

  1859. <p>Note that accessing the ctypes object through the wrapper can be a lot slower

  1860. than accessing the raw ctypes object.</p>

  1861. <div class="versionchanged">

  1862. <p><span class="versionmodified changed">Changed in version 3.5: </span>Synchronized objects support the <a class="reference internal" href="../glossary.html#term-context-manager"><span class="xref std std-term">context manager</span></a> protocol.</p>

  1863. </div>

  1864. </dd></dl>

  1865. 

  1866. <p>The table below compares the syntax for creating shared ctypes objects from

  1867. shared memory with the normal ctypes syntax.  (In the table <code class="docutils literal notranslate"><span class="pre">MyStruct</span></code> is some

  1868. subclass of <a class="reference internal" href="ctypes.html#ctypes.Structure" title="ctypes.Structure"><code class="xref py py-class docutils literal notranslate"><span class="pre">ctypes.Structure</span></code></a>.)</p>

  1869. <table class="docutils align-default">

  1870. <thead>

  1871. <tr class="row-odd"><th class="head"><p>ctypes</p></th>

  1872. <th class="head"><p>sharedctypes using type</p></th>

  1873. <th class="head"><p>sharedctypes using typecode</p></th>

  1874. </tr>

  1875. </thead>

  1876. <tbody>

  1877. <tr class="row-even"><td><p>c_double(2.4)</p></td>

  1878. <td><p>RawValue(c_double, 2.4)</p></td>

  1879. <td><p>RawValue(‘d’, 2.4)</p></td>

  1880. </tr>

  1881. <tr class="row-odd"><td><p>MyStruct(4, 6)</p></td>

  1882. <td><p>RawValue(MyStruct, 4, 6)</p></td>

  1883. <td></td>

  1884. </tr>

  1885. <tr class="row-even"><td><p>(c_short * 7)()</p></td>

  1886. <td><p>RawArray(c_short, 7)</p></td>

  1887. <td><p>RawArray(‘h’, 7)</p></td>

  1888. </tr>

  1889. <tr class="row-odd"><td><p>(c_int * 3)(9, 2, 8)</p></td>

  1890. <td><p>RawArray(c_int, (9, 2, 8))</p></td>

  1891. <td><p>RawArray(‘i’, (9, 2, 8))</p></td>

  1892. </tr>

  1893. </tbody>

  1894. </table>

  1895. <p>Below is an example where a number of ctypes objects are modified by a child

  1896. process:</p>

  1897. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Lock</span>

  1898. <span class="kn">from</span> <span class="nn">multiprocessing.sharedctypes</span> <span class="kn">import</span> <span class="n">Value</span><span class="p">,</span> <span class="n">Array</span>

  1899. <span class="kn">from</span> <span class="nn">ctypes</span> <span class="kn">import</span> <span class="n">Structure</span><span class="p">,</span> <span class="n">c_double</span>

  1900. 

  1901. <span class="k">class</span> <span class="nc">Point</span><span class="p">(</span><span class="n">Structure</span><span class="p">):</span>

  1902.     <span class="n">_fields_</span> <span class="o">=</span> <span class="p">[(</span><span class="s1">&#39;x&#39;</span><span class="p">,</span> <span class="n">c_double</span><span class="p">),</span> <span class="p">(</span><span class="s1">&#39;y&#39;</span><span class="p">,</span> <span class="n">c_double</span><span class="p">)]</span>

  1903. 

  1904. <span class="k">def</span> <span class="nf">modify</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">s</span><span class="p">,</span> <span class="n">A</span><span class="p">):</span>

  1905.     <span class="n">n</span><span class="o">.</span><span class="n">value</span> <span class="o">**=</span> <span class="mi">2</span>

  1906.     <span class="n">x</span><span class="o">.</span><span class="n">value</span> <span class="o">**=</span> <span class="mi">2</span>

  1907.     <span class="n">s</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">s</span><span class="o">.</span><span class="n">value</span><span class="o">.</span><span class="n">upper</span><span class="p">()</span>

  1908.     <span class="k">for</span> <span class="n">a</span> <span class="ow">in</span> <span class="n">A</span><span class="p">:</span>

  1909.         <span class="n">a</span><span class="o">.</span><span class="n">x</span> <span class="o">**=</span> <span class="mi">2</span>

  1910.         <span class="n">a</span><span class="o">.</span><span class="n">y</span> <span class="o">**=</span> <span class="mi">2</span>

  1911. 

  1912. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  1913.     <span class="n">lock</span> <span class="o">=</span> <span class="n">Lock</span><span class="p">()</span>

  1914. 

  1915.     <span class="n">n</span> <span class="o">=</span> <span class="n">Value</span><span class="p">(</span><span class="s1">&#39;i&#39;</span><span class="p">,</span> <span class="mi">7</span><span class="p">)</span>

  1916.     <span class="n">x</span> <span class="o">=</span> <span class="n">Value</span><span class="p">(</span><span class="n">c_double</span><span class="p">,</span> <span class="mf">1.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">,</span> <span class="n">lock</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>

  1917.     <span class="n">s</span> <span class="o">=</span> <span class="n">Array</span><span class="p">(</span><span class="s1">&#39;c&#39;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;hello world&#39;</span><span class="p">,</span> <span class="n">lock</span><span class="o">=</span><span class="n">lock</span><span class="p">)</span>

  1918.     <span class="n">A</span> <span class="o">=</span> <span class="n">Array</span><span class="p">(</span><span class="n">Point</span><span class="p">,</span> <span class="p">[(</span><span class="mf">1.875</span><span class="p">,</span><span class="o">-</span><span class="mf">6.25</span><span class="p">),</span> <span class="p">(</span><span class="o">-</span><span class="mf">5.75</span><span class="p">,</span><span class="mf">2.0</span><span class="p">),</span> <span class="p">(</span><span class="mf">2.375</span><span class="p">,</span><span class="mf">9.5</span><span class="p">)],</span> <span class="n">lock</span><span class="o">=</span><span class="n">lock</span><span class="p">)</span>

  1919. 

  1920.     <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">modify</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">s</span><span class="p">,</span> <span class="n">A</span><span class="p">))</span>

  1921.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  1922.     <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  1923. 

  1924.     <span class="nb">print</span><span class="p">(</span><span class="n">n</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>

  1925.     <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>

  1926.     <span class="nb">print</span><span class="p">(</span><span class="n">s</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>

  1927.     <span class="nb">print</span><span class="p">([(</span><span class="n">a</span><span class="o">.</span><span class="n">x</span><span class="p">,</span> <span class="n">a</span><span class="o">.</span><span class="n">y</span><span class="p">)</span> <span class="k">for</span> <span class="n">a</span> <span class="ow">in</span> <span class="n">A</span><span class="p">])</span>

  1928. </pre></div>

  1929. </div>

  1930. <p>The results printed are</p>

  1931. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>49

  1932. 0.1111111111111111

  1933. HELLO WORLD

  1934. [(3.515625, 39.0625), (33.0625, 4.0), (5.640625, 90.25)]

  1935. </pre></div>

  1936. </div>

  1937. </section>

  1938. </section>

  1939. <section id="managers">

  1940. <span id="multiprocessing-managers"></span><h3>Managers<a class="headerlink" href="#managers" title="Link to this heading">¶</a></h3>

  1941. <p>Managers provide a way to create data which can be shared between different

  1942. processes, including sharing over a network between processes running on

  1943. different machines. A manager object controls a server process which manages

  1944. <em>shared objects</em>.  Other processes can access the shared objects by using

  1945. proxies.</p>

  1946. <dl class="py function">

  1947. <dt class="sig sig-object py" id="multiprocessing.Manager">

  1948. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">Manager</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.Manager" title="Link to this definition">¶</a></dt>

  1949. <dd><p>Returns a started <a class="reference internal" href="#multiprocessing.managers.SyncManager" title="multiprocessing.managers.SyncManager"><code class="xref py py-class docutils literal notranslate"><span class="pre">SyncManager</span></code></a> object which

  1950. can be used for sharing objects between processes.  The returned manager

  1951. object corresponds to a spawned child process and has methods which will

  1952. create shared objects and return corresponding proxies.</p>

  1953. </dd></dl>

  1954. 

  1955. <p id="module-multiprocessing.managers">Manager processes will be shutdown as soon as they are garbage collected or

  1956. their parent process exits.  The manager classes are defined in the

  1957. <a class="reference internal" href="#module-multiprocessing.managers" title="multiprocessing.managers: Share data between process with shared objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.managers</span></code></a> module:</p>

  1958. <dl class="py class">

  1959. <dt class="sig sig-object py" id="multiprocessing.managers.BaseManager">

  1960. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.managers.</span></span><span class="sig-name descname"><span class="pre">BaseManager</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">address</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">authkey</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">serializer</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">'pickle'</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">ctx</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">shutdown_timeout</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">1.0</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseManager" title="Link to this definition">¶</a></dt>

  1961. <dd><p>Create a BaseManager object.</p>

  1962. <p>Once created one should call <a class="reference internal" href="#multiprocessing.managers.BaseManager.start" title="multiprocessing.managers.BaseManager.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">start()</span></code></a> or <code class="docutils literal notranslate"><span class="pre">get_server().serve_forever()</span></code> to ensure

  1963. that the manager object refers to a started manager process.</p>

  1964. <p><em>address</em> is the address on which the manager process listens for new

  1965. connections.  If <em>address</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> then an arbitrary one is chosen.</p>

  1966. <p><em>authkey</em> is the authentication key which will be used to check the

  1967. validity of incoming connections to the server process.  If

  1968. <em>authkey</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> then <code class="docutils literal notranslate"><span class="pre">current_process().authkey</span></code> is used.

  1969. Otherwise <em>authkey</em> is used and it must be a byte string.</p>

  1970. <p><em>serializer</em> must be <code class="docutils literal notranslate"><span class="pre">'pickle'</span></code> (use <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> serialization) or

  1971. <code class="docutils literal notranslate"><span class="pre">'xmlrpclib'</span></code> (use <a class="reference internal" href="xmlrpc.client.html#module-xmlrpc.client" title="xmlrpc.client: XML-RPC client access."><code class="xref py py-mod docutils literal notranslate"><span class="pre">xmlrpc.client</span></code></a> serialization).</p>

  1972. <p><em>ctx</em> is a context object, or <code class="docutils literal notranslate"><span class="pre">None</span></code> (use the current context). See the

  1973. <code class="xref py py-func docutils literal notranslate"><span class="pre">get_context()</span></code> function.</p>

  1974. <p><em>shutdown_timeout</em> is a timeout in seconds used to wait until the process

  1975. used by the manager completes in the <a class="reference internal" href="#multiprocessing.managers.BaseManager.shutdown" title="multiprocessing.managers.BaseManager.shutdown"><code class="xref py py-meth docutils literal notranslate"><span class="pre">shutdown()</span></code></a> method. If the

  1976. shutdown times out, the process is terminated. If terminating the process

  1977. also times out, the process is killed.</p>

  1978. <div class="versionchanged">

  1979. <p><span class="versionmodified changed">Changed in version 3.11: </span>Added the <em>shutdown_timeout</em> parameter.</p>

  1980. </div>

  1981. <dl class="py method">

  1982. <dt class="sig sig-object py" id="multiprocessing.managers.BaseManager.start">

  1983. <span class="sig-name descname"><span class="pre">start</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">initializer</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">initargs</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseManager.start" title="Link to this definition">¶</a></dt>

  1984. <dd><p>Start a subprocess to start the manager.  If <em>initializer</em> is not <code class="docutils literal notranslate"><span class="pre">None</span></code>

  1985. then the subprocess will call <code class="docutils literal notranslate"><span class="pre">initializer(*initargs)</span></code> when it starts.</p>

  1986. </dd></dl>

  1987. 

  1988. <dl class="py method">

  1989. <dt class="sig sig-object py" id="multiprocessing.managers.BaseManager.get_server">

  1990. <span class="sig-name descname"><span class="pre">get_server</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseManager.get_server" title="Link to this definition">¶</a></dt>

  1991. <dd><p>Returns a <code class="xref py py-class docutils literal notranslate"><span class="pre">Server</span></code> object which represents the actual server under

  1992. the control of the Manager. The <code class="xref py py-class docutils literal notranslate"><span class="pre">Server</span></code> object supports the

  1993. <code class="xref py py-meth docutils literal notranslate"><span class="pre">serve_forever()</span></code> method:</p>

  1994. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing.managers</span> <span class="kn">import</span> <span class="n">BaseManager</span>

  1995. <span class="gp">&gt;&gt;&gt; </span><span class="n">manager</span> <span class="o">=</span> <span class="n">BaseManager</span><span class="p">(</span><span class="n">address</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;&#39;</span><span class="p">,</span> <span class="mi">50000</span><span class="p">),</span> <span class="n">authkey</span><span class="o">=</span><span class="sa">b</span><span class="s1">&#39;abc&#39;</span><span class="p">)</span>

  1996. <span class="gp">&gt;&gt;&gt; </span><span class="n">server</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">get_server</span><span class="p">()</span>

  1997. <span class="gp">&gt;&gt;&gt; </span><span class="n">server</span><span class="o">.</span><span class="n">serve_forever</span><span class="p">()</span>

  1998. </pre></div>

  1999. </div>

  2000. <p><code class="xref py py-class docutils literal notranslate"><span class="pre">Server</span></code> additionally has an <a class="reference internal" href="#multiprocessing.managers.BaseManager.address" title="multiprocessing.managers.BaseManager.address"><code class="xref py py-attr docutils literal notranslate"><span class="pre">address</span></code></a> attribute.</p>

  2001. </dd></dl>

  2002. 

  2003. <dl class="py method">

  2004. <dt class="sig sig-object py" id="multiprocessing.managers.BaseManager.connect">

  2005. <span class="sig-name descname"><span class="pre">connect</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseManager.connect" title="Link to this definition">¶</a></dt>

  2006. <dd><p>Connect a local manager object to a remote manager process:</p>

  2007. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing.managers</span> <span class="kn">import</span> <span class="n">BaseManager</span>

  2008. <span class="gp">&gt;&gt;&gt; </span><span class="n">m</span> <span class="o">=</span> <span class="n">BaseManager</span><span class="p">(</span><span class="n">address</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;127.0.0.1&#39;</span><span class="p">,</span> <span class="mi">50000</span><span class="p">),</span> <span class="n">authkey</span><span class="o">=</span><span class="sa">b</span><span class="s1">&#39;abc&#39;</span><span class="p">)</span>

  2009. <span class="gp">&gt;&gt;&gt; </span><span class="n">m</span><span class="o">.</span><span class="n">connect</span><span class="p">()</span>

  2010. </pre></div>

  2011. </div>

  2012. </dd></dl>

  2013. 

  2014. <dl class="py method">

  2015. <dt class="sig sig-object py" id="multiprocessing.managers.BaseManager.shutdown">

  2016. <span class="sig-name descname"><span class="pre">shutdown</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseManager.shutdown" title="Link to this definition">¶</a></dt>

  2017. <dd><p>Stop the process used by the manager.  This is only available if

  2018. <a class="reference internal" href="#multiprocessing.managers.BaseManager.start" title="multiprocessing.managers.BaseManager.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">start()</span></code></a> has been used to start the server process.</p>

  2019. <p>This can be called multiple times.</p>

  2020. </dd></dl>

  2021. 

  2022. <dl class="py method">

  2023. <dt class="sig sig-object py" id="multiprocessing.managers.BaseManager.register">

  2024. <span class="sig-name descname"><span class="pre">register</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">typeid</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">callable</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">proxytype</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">exposed</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">method_to_typeid</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">create_method</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseManager.register" title="Link to this definition">¶</a></dt>

  2025. <dd><p>A classmethod which can be used for registering a type or callable with

  2026. the manager class.</p>

  2027. <p><em>typeid</em> is a “type identifier” which is used to identify a particular

  2028. type of shared object.  This must be a string.</p>

  2029. <p><em>callable</em> is a callable used for creating objects for this type

  2030. identifier.  If a manager instance will be connected to the

  2031. server using the <a class="reference internal" href="#multiprocessing.managers.BaseManager.connect" title="multiprocessing.managers.BaseManager.connect"><code class="xref py py-meth docutils literal notranslate"><span class="pre">connect()</span></code></a> method, or if the

  2032. <em>create_method</em> argument is <code class="docutils literal notranslate"><span class="pre">False</span></code> then this can be left as

  2033. <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>

  2034. <p><em>proxytype</em> is a subclass of <a class="reference internal" href="#multiprocessing.managers.BaseProxy" title="multiprocessing.managers.BaseProxy"><code class="xref py py-class docutils literal notranslate"><span class="pre">BaseProxy</span></code></a> which is used to create

  2035. proxies for shared objects with this <em>typeid</em>.  If <code class="docutils literal notranslate"><span class="pre">None</span></code> then a proxy

  2036. class is created automatically.</p>

  2037. <p><em>exposed</em> is used to specify a sequence of method names which proxies for

  2038. this typeid should be allowed to access using

  2039. <a class="reference internal" href="#multiprocessing.managers.BaseProxy._callmethod" title="multiprocessing.managers.BaseProxy._callmethod"><code class="xref py py-meth docutils literal notranslate"><span class="pre">BaseProxy._callmethod()</span></code></a>.  (If <em>exposed</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> then

  2040. <code class="xref py py-attr docutils literal notranslate"><span class="pre">proxytype._exposed_</span></code> is used instead if it exists.)  In the case

  2041. where no exposed list is specified, all “public methods” of the shared

  2042. object will be accessible.  (Here a “public method” means any attribute

  2043. which has a <a class="reference internal" href="../reference/datamodel.html#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__call__()</span></code></a> method and whose name does not begin

  2044. with <code class="docutils literal notranslate"><span class="pre">'_'</span></code>.)</p>

  2045. <p><em>method_to_typeid</em> is a mapping used to specify the return type of those

  2046. exposed methods which should return a proxy.  It maps method names to

  2047. typeid strings.  (If <em>method_to_typeid</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> then

  2048. <code class="xref py py-attr docutils literal notranslate"><span class="pre">proxytype._method_to_typeid_</span></code> is used instead if it exists.)  If a

  2049. method’s name is not a key of this mapping or if the mapping is <code class="docutils literal notranslate"><span class="pre">None</span></code>

  2050. then the object returned by the method will be copied by value.</p>

  2051. <p><em>create_method</em> determines whether a method should be created with name

  2052. <em>typeid</em> which can be used to tell the server process to create a new

  2053. shared object and return a proxy for it.  By default it is <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  2054. </dd></dl>

  2055. 

  2056. <p><a class="reference internal" href="#multiprocessing.managers.BaseManager" title="multiprocessing.managers.BaseManager"><code class="xref py py-class docutils literal notranslate"><span class="pre">BaseManager</span></code></a> instances also have one read-only property:</p>

  2057. <dl class="py attribute">

  2058. <dt class="sig sig-object py" id="multiprocessing.managers.BaseManager.address">

  2059. <span class="sig-name descname"><span class="pre">address</span></span><a class="headerlink" href="#multiprocessing.managers.BaseManager.address" title="Link to this definition">¶</a></dt>

  2060. <dd><p>The address used by the manager.</p>

  2061. </dd></dl>

  2062. 

  2063. <div class="versionchanged">

  2064. <p><span class="versionmodified changed">Changed in version 3.3: </span>Manager objects support the context management protocol – see

  2065. <a class="reference internal" href="stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a>.  <a class="reference internal" href="stdtypes.html#contextmanager.__enter__" title="contextmanager.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> starts the

  2066. server process (if it has not already started) and then returns the

  2067. manager object.  <a class="reference internal" href="stdtypes.html#contextmanager.__exit__" title="contextmanager.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> calls <a class="reference internal" href="#multiprocessing.managers.BaseManager.shutdown" title="multiprocessing.managers.BaseManager.shutdown"><code class="xref py py-meth docutils literal notranslate"><span class="pre">shutdown()</span></code></a>.</p>

  2068. <p>In previous versions <a class="reference internal" href="stdtypes.html#contextmanager.__enter__" title="contextmanager.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> did not start the

  2069. manager’s server process if it was not already started.</p>

  2070. </div>

  2071. </dd></dl>

  2072. 

  2073. <dl class="py class">

  2074. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager">

  2075. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.managers.</span></span><span class="sig-name descname"><span class="pre">SyncManager</span></span><a class="headerlink" href="#multiprocessing.managers.SyncManager" title="Link to this definition">¶</a></dt>

  2076. <dd><p>A subclass of <a class="reference internal" href="#multiprocessing.managers.BaseManager" title="multiprocessing.managers.BaseManager"><code class="xref py py-class docutils literal notranslate"><span class="pre">BaseManager</span></code></a> which can be used for the synchronization

  2077. of processes.  Objects of this type are returned by

  2078. <a class="reference internal" href="#multiprocessing.Manager" title="multiprocessing.Manager"><code class="xref py py-func docutils literal notranslate"><span class="pre">multiprocessing.Manager()</span></code></a>.</p>

  2079. <p>Its methods create and return <a class="reference internal" href="#multiprocessing-proxy-objects"><span class="std std-ref">Proxy Objects</span></a> for a

  2080. number of commonly used data types to be synchronized across processes.

  2081. This notably includes shared lists and dictionaries.</p>

  2082. <dl class="py method">

  2083. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.Barrier">

  2084. <span class="sig-name descname"><span class="pre">Barrier</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">parties</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">action</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.Barrier" title="Link to this definition">¶</a></dt>

  2085. <dd><p>Create a shared <a class="reference internal" href="threading.html#threading.Barrier" title="threading.Barrier"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Barrier</span></code></a> object and return a

  2086. proxy for it.</p>

  2087. <div class="versionadded">

  2088. <p><span class="versionmodified added">New in version 3.3.</span></p>

  2089. </div>

  2090. </dd></dl>

  2091. 

  2092. <dl class="py method">

  2093. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.BoundedSemaphore">

  2094. <span class="sig-name descname"><span class="pre">BoundedSemaphore</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.BoundedSemaphore" title="Link to this definition">¶</a></dt>

  2095. <dd><p>Create a shared <a class="reference internal" href="threading.html#threading.BoundedSemaphore" title="threading.BoundedSemaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.BoundedSemaphore</span></code></a> object and return a

  2096. proxy for it.</p>

  2097. </dd></dl>

  2098. 

  2099. <dl class="py method">

  2100. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.Condition">

  2101. <span class="sig-name descname"><span class="pre">Condition</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">lock</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.Condition" title="Link to this definition">¶</a></dt>

  2102. <dd><p>Create a shared <a class="reference internal" href="threading.html#threading.Condition" title="threading.Condition"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Condition</span></code></a> object and return a proxy for

  2103. it.</p>

  2104. <p>If <em>lock</em> is supplied then it should be a proxy for a

  2105. <a class="reference internal" href="threading.html#threading.Lock" title="threading.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Lock</span></code></a> or <a class="reference internal" href="threading.html#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.RLock</span></code></a> object.</p>

  2106. <div class="versionchanged">

  2107. <p><span class="versionmodified changed">Changed in version 3.3: </span>The <a class="reference internal" href="threading.html#threading.Condition.wait_for" title="threading.Condition.wait_for"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait_for()</span></code></a> method was added.</p>

  2108. </div>

  2109. </dd></dl>

  2110. 

  2111. <dl class="py method">

  2112. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.Event">

  2113. <span class="sig-name descname"><span class="pre">Event</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.Event" title="Link to this definition">¶</a></dt>

  2114. <dd><p>Create a shared <a class="reference internal" href="threading.html#threading.Event" title="threading.Event"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Event</span></code></a> object and return a proxy for it.</p>

  2115. </dd></dl>

  2116. 

  2117. <dl class="py method">

  2118. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.Lock">

  2119. <span class="sig-name descname"><span class="pre">Lock</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.Lock" title="Link to this definition">¶</a></dt>

  2120. <dd><p>Create a shared <a class="reference internal" href="threading.html#threading.Lock" title="threading.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Lock</span></code></a> object and return a proxy for it.</p>

  2121. </dd></dl>

  2122. 

  2123. <dl class="py method">

  2124. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.Namespace">

  2125. <span class="sig-name descname"><span class="pre">Namespace</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.Namespace" title="Link to this definition">¶</a></dt>

  2126. <dd><p>Create a shared <a class="reference internal" href="#multiprocessing.managers.Namespace" title="multiprocessing.managers.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a> object and return a proxy for it.</p>

  2127. </dd></dl>

  2128. 

  2129. <dl class="py method">

  2130. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.Queue">

  2131. <span class="sig-name descname"><span class="pre">Queue</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">maxsize</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.Queue" title="Link to this definition">¶</a></dt>

  2132. <dd><p>Create a shared <a class="reference internal" href="queue.html#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">queue.Queue</span></code></a> object and return a proxy for it.</p>

  2133. </dd></dl>

  2134. 

  2135. <dl class="py method">

  2136. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.RLock">

  2137. <span class="sig-name descname"><span class="pre">RLock</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.RLock" title="Link to this definition">¶</a></dt>

  2138. <dd><p>Create a shared <a class="reference internal" href="threading.html#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.RLock</span></code></a> object and return a proxy for it.</p>

  2139. </dd></dl>

  2140. 

  2141. <dl class="py method">

  2142. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.Semaphore">

  2143. <span class="sig-name descname"><span class="pre">Semaphore</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.Semaphore" title="Link to this definition">¶</a></dt>

  2144. <dd><p>Create a shared <a class="reference internal" href="threading.html#threading.Semaphore" title="threading.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Semaphore</span></code></a> object and return a proxy for

  2145. it.</p>

  2146. </dd></dl>

  2147. 

  2148. <dl class="py method">

  2149. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.Array">

  2150. <span class="sig-name descname"><span class="pre">Array</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">typecode</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">sequence</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.Array" title="Link to this definition">¶</a></dt>

  2151. <dd><p>Create an array and return a proxy for it.</p>

  2152. </dd></dl>

  2153. 

  2154. <dl class="py method">

  2155. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.Value">

  2156. <span class="sig-name descname"><span class="pre">Value</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">typecode</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.Value" title="Link to this definition">¶</a></dt>

  2157. <dd><p>Create an object with a writable <code class="docutils literal notranslate"><span class="pre">value</span></code> attribute and return a proxy

  2158. for it.</p>

  2159. </dd></dl>

  2160. 

  2161. <dl class="py method">

  2162. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.dict">

  2163. <span class="sig-name descname"><span class="pre">dict</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.dict" title="Link to this definition">¶</a></dt>

  2164. <dt class="sig sig-object py">

  2165. <span class="sig-name descname"><span class="pre">dict</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">mapping</span></span></em><span class="sig-paren">)</span></dt>

  2166. <dt class="sig sig-object py">

  2167. <span class="sig-name descname"><span class="pre">dict</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">sequence</span></span></em><span class="sig-paren">)</span></dt>

  2168. <dd><p>Create a shared <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a> object and return a proxy for it.</p>

  2169. </dd></dl>

  2170. 

  2171. <dl class="py method">

  2172. <dt class="sig sig-object py" id="multiprocessing.managers.SyncManager.list">

  2173. <span class="sig-name descname"><span class="pre">list</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.SyncManager.list" title="Link to this definition">¶</a></dt>

  2174. <dt class="sig sig-object py">

  2175. <span class="sig-name descname"><span class="pre">list</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">sequence</span></span></em><span class="sig-paren">)</span></dt>

  2176. <dd><p>Create a shared <a class="reference internal" href="stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate"><span class="pre">list</span></code></a> object and return a proxy for it.</p>

  2177. </dd></dl>

  2178. 

  2179. <div class="versionchanged">

  2180. <p><span class="versionmodified changed">Changed in version 3.6: </span>Shared objects are capable of being nested.  For example, a shared

  2181. container object such as a shared list can contain other shared objects

  2182. which will all be managed and synchronized by the <a class="reference internal" href="#multiprocessing.managers.SyncManager" title="multiprocessing.managers.SyncManager"><code class="xref py py-class docutils literal notranslate"><span class="pre">SyncManager</span></code></a>.</p>

  2183. </div>

  2184. </dd></dl>

  2185. 

  2186. <dl class="py class">

  2187. <dt class="sig sig-object py" id="multiprocessing.managers.Namespace">

  2188. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.managers.</span></span><span class="sig-name descname"><span class="pre">Namespace</span></span><a class="headerlink" href="#multiprocessing.managers.Namespace" title="Link to this definition">¶</a></dt>

  2189. <dd><p>A type that can register with <a class="reference internal" href="#multiprocessing.managers.SyncManager" title="multiprocessing.managers.SyncManager"><code class="xref py py-class docutils literal notranslate"><span class="pre">SyncManager</span></code></a>.</p>

  2190. <p>A namespace object has no public methods, but does have writable attributes.

  2191. Its representation shows the values of its attributes.</p>

  2192. <p>However, when using a proxy for a namespace object, an attribute beginning

  2193. with <code class="docutils literal notranslate"><span class="pre">'_'</span></code> will be an attribute of the proxy and not an attribute of the

  2194. referent:</p>

  2195. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">mp_context</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">get_context</span><span class="p">(</span><span class="s1">&#39;spawn&#39;</span><span class="p">)</span>

  2196. <span class="gp">&gt;&gt;&gt; </span><span class="n">manager</span> <span class="o">=</span> <span class="n">mp_context</span><span class="o">.</span><span class="n">Manager</span><span class="p">()</span>

  2197. <span class="gp">&gt;&gt;&gt; </span><span class="n">Global</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">Namespace</span><span class="p">()</span>

  2198. <span class="gp">&gt;&gt;&gt; </span><span class="n">Global</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mi">10</span>

  2199. <span class="gp">&gt;&gt;&gt; </span><span class="n">Global</span><span class="o">.</span><span class="n">y</span> <span class="o">=</span> <span class="s1">&#39;hello&#39;</span>

  2200. <span class="gp">&gt;&gt;&gt; </span><span class="n">Global</span><span class="o">.</span><span class="n">_z</span> <span class="o">=</span> <span class="mf">12.3</span>    <span class="c1"># this is an attribute of the proxy</span>

  2201. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">Global</span><span class="p">)</span>

  2202. <span class="go">Namespace(x=10, y=&#39;hello&#39;)</span>

  2203. </pre></div>

  2204. </div>

  2205. </dd></dl>

  2206. 

  2207. <section id="customized-managers">

  2208. <h4>Customized managers<a class="headerlink" href="#customized-managers" title="Link to this heading">¶</a></h4>

  2209. <p>To create one’s own manager, one creates a subclass of <a class="reference internal" href="#multiprocessing.managers.BaseManager" title="multiprocessing.managers.BaseManager"><code class="xref py py-class docutils literal notranslate"><span class="pre">BaseManager</span></code></a> and

  2210. uses the <a class="reference internal" href="#multiprocessing.managers.BaseManager.register" title="multiprocessing.managers.BaseManager.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code></a> classmethod to register new types or

  2211. callables with the manager class.  For example:</p>

  2212. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing.managers</span> <span class="kn">import</span> <span class="n">BaseManager</span>

  2213. 

  2214. <span class="k">class</span> <span class="nc">MathsClass</span><span class="p">:</span>

  2215.     <span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>

  2216.         <span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span>

  2217.     <span class="k">def</span> <span class="nf">mul</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>

  2218.         <span class="k">return</span> <span class="n">x</span> <span class="o">*</span> <span class="n">y</span>

  2219. 

  2220. <span class="k">class</span> <span class="nc">MyManager</span><span class="p">(</span><span class="n">BaseManager</span><span class="p">):</span>

  2221.     <span class="k">pass</span>

  2222. 

  2223. <span class="n">MyManager</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">&#39;Maths&#39;</span><span class="p">,</span> <span class="n">MathsClass</span><span class="p">)</span>

  2224. 

  2225. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  2226.     <span class="k">with</span> <span class="n">MyManager</span><span class="p">()</span> <span class="k">as</span> <span class="n">manager</span><span class="p">:</span>

  2227.         <span class="n">maths</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">Maths</span><span class="p">()</span>

  2228.         <span class="nb">print</span><span class="p">(</span><span class="n">maths</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="mi">4</span><span class="p">,</span> <span class="mi">3</span><span class="p">))</span>         <span class="c1"># prints 7</span>

  2229.         <span class="nb">print</span><span class="p">(</span><span class="n">maths</span><span class="o">.</span><span class="n">mul</span><span class="p">(</span><span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">))</span>         <span class="c1"># prints 56</span>

  2230. </pre></div>

  2231. </div>

  2232. </section>

  2233. <section id="using-a-remote-manager">

  2234. <h4>Using a remote manager<a class="headerlink" href="#using-a-remote-manager" title="Link to this heading">¶</a></h4>

  2235. <p>It is possible to run a manager server on one machine and have clients use it

  2236. from other machines (assuming that the firewalls involved allow it).</p>

  2237. <p>Running the following commands creates a server for a single shared queue which

  2238. remote clients can access:</p>

  2239. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing.managers</span> <span class="kn">import</span> <span class="n">BaseManager</span>

  2240. <span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">queue</span> <span class="kn">import</span> <span class="n">Queue</span>

  2241. <span class="gp">&gt;&gt;&gt; </span><span class="n">queue</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span>

  2242. <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">QueueManager</span><span class="p">(</span><span class="n">BaseManager</span><span class="p">):</span> <span class="k">pass</span>

  2243. <span class="gp">&gt;&gt;&gt; </span><span class="n">QueueManager</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">&#39;get_queue&#39;</span><span class="p">,</span> <span class="nb">callable</span><span class="o">=</span><span class="k">lambda</span><span class="p">:</span><span class="n">queue</span><span class="p">)</span>

  2244. <span class="gp">&gt;&gt;&gt; </span><span class="n">m</span> <span class="o">=</span> <span class="n">QueueManager</span><span class="p">(</span><span class="n">address</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;&#39;</span><span class="p">,</span> <span class="mi">50000</span><span class="p">),</span> <span class="n">authkey</span><span class="o">=</span><span class="sa">b</span><span class="s1">&#39;abracadabra&#39;</span><span class="p">)</span>

  2245. <span class="gp">&gt;&gt;&gt; </span><span class="n">s</span> <span class="o">=</span> <span class="n">m</span><span class="o">.</span><span class="n">get_server</span><span class="p">()</span>

  2246. <span class="gp">&gt;&gt;&gt; </span><span class="n">s</span><span class="o">.</span><span class="n">serve_forever</span><span class="p">()</span>

  2247. </pre></div>

  2248. </div>

  2249. <p>One client can access the server as follows:</p>

  2250. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing.managers</span> <span class="kn">import</span> <span class="n">BaseManager</span>

  2251. <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">QueueManager</span><span class="p">(</span><span class="n">BaseManager</span><span class="p">):</span> <span class="k">pass</span>

  2252. <span class="gp">&gt;&gt;&gt; </span><span class="n">QueueManager</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">&#39;get_queue&#39;</span><span class="p">)</span>

  2253. <span class="gp">&gt;&gt;&gt; </span><span class="n">m</span> <span class="o">=</span> <span class="n">QueueManager</span><span class="p">(</span><span class="n">address</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;foo.bar.org&#39;</span><span class="p">,</span> <span class="mi">50000</span><span class="p">),</span> <span class="n">authkey</span><span class="o">=</span><span class="sa">b</span><span class="s1">&#39;abracadabra&#39;</span><span class="p">)</span>

  2254. <span class="gp">&gt;&gt;&gt; </span><span class="n">m</span><span class="o">.</span><span class="n">connect</span><span class="p">()</span>

  2255. <span class="gp">&gt;&gt;&gt; </span><span class="n">queue</span> <span class="o">=</span> <span class="n">m</span><span class="o">.</span><span class="n">get_queue</span><span class="p">()</span>

  2256. <span class="gp">&gt;&gt;&gt; </span><span class="n">queue</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="s1">&#39;hello&#39;</span><span class="p">)</span>

  2257. </pre></div>

  2258. </div>

  2259. <p>Another client can also use it:</p>

  2260. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing.managers</span> <span class="kn">import</span> <span class="n">BaseManager</span>

  2261. <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">QueueManager</span><span class="p">(</span><span class="n">BaseManager</span><span class="p">):</span> <span class="k">pass</span>

  2262. <span class="gp">&gt;&gt;&gt; </span><span class="n">QueueManager</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">&#39;get_queue&#39;</span><span class="p">)</span>

  2263. <span class="gp">&gt;&gt;&gt; </span><span class="n">m</span> <span class="o">=</span> <span class="n">QueueManager</span><span class="p">(</span><span class="n">address</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;foo.bar.org&#39;</span><span class="p">,</span> <span class="mi">50000</span><span class="p">),</span> <span class="n">authkey</span><span class="o">=</span><span class="sa">b</span><span class="s1">&#39;abracadabra&#39;</span><span class="p">)</span>

  2264. <span class="gp">&gt;&gt;&gt; </span><span class="n">m</span><span class="o">.</span><span class="n">connect</span><span class="p">()</span>

  2265. <span class="gp">&gt;&gt;&gt; </span><span class="n">queue</span> <span class="o">=</span> <span class="n">m</span><span class="o">.</span><span class="n">get_queue</span><span class="p">()</span>

  2266. <span class="gp">&gt;&gt;&gt; </span><span class="n">queue</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>

  2267. <span class="go">&#39;hello&#39;</span>

  2268. </pre></div>

  2269. </div>

  2270. <p>Local processes can also access that queue, using the code from above on the

  2271. client to access it remotely:</p>

  2272. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Queue</span>

  2273. <span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">multiprocessing.managers</span> <span class="kn">import</span> <span class="n">BaseManager</span>

  2274. <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">Worker</span><span class="p">(</span><span class="n">Process</span><span class="p">):</span>

  2275. <span class="gp">... </span>    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">q</span><span class="p">):</span>

  2276. <span class="gp">... </span>        <span class="bp">self</span><span class="o">.</span><span class="n">q</span> <span class="o">=</span> <span class="n">q</span>

  2277. <span class="gp">... </span>        <span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__init__</span><span class="p">()</span>

  2278. <span class="gp">... </span>    <span class="k">def</span> <span class="nf">run</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  2279. <span class="gp">... </span>        <span class="bp">self</span><span class="o">.</span><span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="s1">&#39;local hello&#39;</span><span class="p">)</span>

  2280. <span class="gp">...</span>

  2281. <span class="gp">&gt;&gt;&gt; </span><span class="n">queue</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span>

  2282. <span class="gp">&gt;&gt;&gt; </span><span class="n">w</span> <span class="o">=</span> <span class="n">Worker</span><span class="p">(</span><span class="n">queue</span><span class="p">)</span>

  2283. <span class="gp">&gt;&gt;&gt; </span><span class="n">w</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  2284. <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">QueueManager</span><span class="p">(</span><span class="n">BaseManager</span><span class="p">):</span> <span class="k">pass</span>

  2285. <span class="gp">...</span>

  2286. <span class="gp">&gt;&gt;&gt; </span><span class="n">QueueManager</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">&#39;get_queue&#39;</span><span class="p">,</span> <span class="nb">callable</span><span class="o">=</span><span class="k">lambda</span><span class="p">:</span> <span class="n">queue</span><span class="p">)</span>

  2287. <span class="gp">&gt;&gt;&gt; </span><span class="n">m</span> <span class="o">=</span> <span class="n">QueueManager</span><span class="p">(</span><span class="n">address</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;&#39;</span><span class="p">,</span> <span class="mi">50000</span><span class="p">),</span> <span class="n">authkey</span><span class="o">=</span><span class="sa">b</span><span class="s1">&#39;abracadabra&#39;</span><span class="p">)</span>

  2288. <span class="gp">&gt;&gt;&gt; </span><span class="n">s</span> <span class="o">=</span> <span class="n">m</span><span class="o">.</span><span class="n">get_server</span><span class="p">()</span>

  2289. <span class="gp">&gt;&gt;&gt; </span><span class="n">s</span><span class="o">.</span><span class="n">serve_forever</span><span class="p">()</span>

  2290. </pre></div>

  2291. </div>

  2292. </section>

  2293. </section>

  2294. <section id="proxy-objects">

  2295. <span id="multiprocessing-proxy-objects"></span><h3>Proxy Objects<a class="headerlink" href="#proxy-objects" title="Link to this heading">¶</a></h3>

  2296. <p>A proxy is an object which <em>refers</em> to a shared object which lives (presumably)

  2297. in a different process.  The shared object is said to be the <em>referent</em> of the

  2298. proxy.  Multiple proxy objects may have the same referent.</p>

  2299. <p>A proxy object has methods which invoke corresponding methods of its referent

  2300. (although not every method of the referent will necessarily be available through

  2301. the proxy).  In this way, a proxy can be used just like its referent can:</p>

  2302. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">mp_context</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">get_context</span><span class="p">(</span><span class="s1">&#39;spawn&#39;</span><span class="p">)</span>

  2303. <span class="gp">&gt;&gt;&gt; </span><span class="n">manager</span> <span class="o">=</span> <span class="n">mp_context</span><span class="o">.</span><span class="n">Manager</span><span class="p">()</span>

  2304. <span class="gp">&gt;&gt;&gt; </span><span class="n">l</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">list</span><span class="p">([</span><span class="n">i</span><span class="o">*</span><span class="n">i</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)])</span>

  2305. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">l</span><span class="p">)</span>

  2306. <span class="go">[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]</span>

  2307. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">l</span><span class="p">))</span>

  2308. <span class="go">&lt;ListProxy object, typeid &#39;list&#39; at 0x...&gt;</span>

  2309. <span class="gp">&gt;&gt;&gt; </span><span class="n">l</span><span class="p">[</span><span class="mi">4</span><span class="p">]</span>

  2310. <span class="go">16</span>

  2311. <span class="gp">&gt;&gt;&gt; </span><span class="n">l</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">5</span><span class="p">]</span>

  2312. <span class="go">[4, 9, 16]</span>

  2313. </pre></div>

  2314. </div>

  2315. <p>Notice that applying <a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str()</span></code></a> to a proxy will return the representation of

  2316. the referent, whereas applying <a class="reference internal" href="functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate"><span class="pre">repr()</span></code></a> will return the representation of

  2317. the proxy.</p>

  2318. <p>An important feature of proxy objects is that they are picklable so they can be

  2319. passed between processes.  As such, a referent can contain

  2320. <a class="reference internal" href="#multiprocessing-proxy-objects"><span class="std std-ref">Proxy Objects</span></a>.  This permits nesting of these managed

  2321. lists, dicts, and other <a class="reference internal" href="#multiprocessing-proxy-objects"><span class="std std-ref">Proxy Objects</span></a>:</p>

  2322. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">a</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">list</span><span class="p">()</span>

  2323. <span class="gp">&gt;&gt;&gt; </span><span class="n">b</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">list</span><span class="p">()</span>

  2324. <span class="gp">&gt;&gt;&gt; </span><span class="n">a</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>         <span class="c1"># referent of a now contains referent of b</span>

  2325. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">)</span>

  2326. <span class="go">[&lt;ListProxy object, typeid &#39;list&#39; at ...&gt;] []</span>

  2327. <span class="gp">&gt;&gt;&gt; </span><span class="n">b</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s1">&#39;hello&#39;</span><span class="p">)</span>

  2328. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="n">b</span><span class="p">)</span>

  2329. <span class="go">[&#39;hello&#39;] [&#39;hello&#39;]</span>

  2330. </pre></div>

  2331. </div>

  2332. <p>Similarly, dict and list proxies may be nested inside one another:</p>

  2333. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">l_outer</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">list</span><span class="p">([</span> <span class="n">manager</span><span class="o">.</span><span class="n">dict</span><span class="p">()</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span> <span class="p">])</span>

  2334. <span class="gp">&gt;&gt;&gt; </span><span class="n">d_first_inner</span> <span class="o">=</span> <span class="n">l_outer</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>

  2335. <span class="gp">&gt;&gt;&gt; </span><span class="n">d_first_inner</span><span class="p">[</span><span class="s1">&#39;a&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>

  2336. <span class="gp">&gt;&gt;&gt; </span><span class="n">d_first_inner</span><span class="p">[</span><span class="s1">&#39;b&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="mi">2</span>

  2337. <span class="gp">&gt;&gt;&gt; </span><span class="n">l_outer</span><span class="p">[</span><span class="mi">1</span><span class="p">][</span><span class="s1">&#39;c&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="mi">3</span>

  2338. <span class="gp">&gt;&gt;&gt; </span><span class="n">l_outer</span><span class="p">[</span><span class="mi">1</span><span class="p">][</span><span class="s1">&#39;z&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="mi">26</span>

  2339. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">l_outer</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>

  2340. <span class="go">{&#39;a&#39;: 1, &#39;b&#39;: 2}</span>

  2341. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">l_outer</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span>

  2342. <span class="go">{&#39;c&#39;: 3, &#39;z&#39;: 26}</span>

  2343. </pre></div>

  2344. </div>

  2345. <p>If standard (non-proxy) <a class="reference internal" href="stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate"><span class="pre">list</span></code></a> or <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a> objects are contained

  2346. in a referent, modifications to those mutable values will not be propagated

  2347. through the manager because the proxy has no way of knowing when the values

  2348. contained within are modified.  However, storing a value in a container proxy

  2349. (which triggers a <code class="docutils literal notranslate"><span class="pre">__setitem__</span></code> on the proxy object) does propagate through

  2350. the manager and so to effectively modify such an item, one could re-assign the

  2351. modified value to the container proxy:</p>

  2352. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># create a list proxy and append a mutable object (a dictionary)</span>

  2353. <span class="n">lproxy</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">list</span><span class="p">()</span>

  2354. <span class="n">lproxy</span><span class="o">.</span><span class="n">append</span><span class="p">({})</span>

  2355. <span class="c1"># now mutate the dictionary</span>

  2356. <span class="n">d</span> <span class="o">=</span> <span class="n">lproxy</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>

  2357. <span class="n">d</span><span class="p">[</span><span class="s1">&#39;a&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>

  2358. <span class="n">d</span><span class="p">[</span><span class="s1">&#39;b&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="mi">2</span>

  2359. <span class="c1"># at this point, the changes to d are not yet synced, but by</span>

  2360. <span class="c1"># updating the dictionary, the proxy is notified of the change</span>

  2361. <span class="n">lproxy</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="n">d</span>

  2362. </pre></div>

  2363. </div>

  2364. <p>This approach is perhaps less convenient than employing nested

  2365. <a class="reference internal" href="#multiprocessing-proxy-objects"><span class="std std-ref">Proxy Objects</span></a> for most use cases but also

  2366. demonstrates a level of control over the synchronization.</p>

  2367. <div class="admonition note">

  2368. <p class="admonition-title">Note</p>

  2369. <p>The proxy types in <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> do nothing to support comparisons

  2370. by value.  So, for instance, we have:</p>

  2371. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">manager</span><span class="o">.</span><span class="n">list</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">])</span> <span class="o">==</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">]</span>

  2372. <span class="go">False</span>

  2373. </pre></div>

  2374. </div>

  2375. <p>One should just use a copy of the referent instead when making comparisons.</p>

  2376. </div>

  2377. <dl class="py class">

  2378. <dt class="sig sig-object py" id="multiprocessing.managers.BaseProxy">

  2379. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.managers.</span></span><span class="sig-name descname"><span class="pre">BaseProxy</span></span><a class="headerlink" href="#multiprocessing.managers.BaseProxy" title="Link to this definition">¶</a></dt>

  2380. <dd><p>Proxy objects are instances of subclasses of <a class="reference internal" href="#multiprocessing.managers.BaseProxy" title="multiprocessing.managers.BaseProxy"><code class="xref py py-class docutils literal notranslate"><span class="pre">BaseProxy</span></code></a>.</p>

  2381. <dl class="py method">

  2382. <dt class="sig sig-object py" id="multiprocessing.managers.BaseProxy._callmethod">

  2383. <span class="sig-name descname"><span class="pre">_callmethod</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">methodname</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">args</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">kwds</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseProxy._callmethod" title="Link to this definition">¶</a></dt>

  2384. <dd><p>Call and return the result of a method of the proxy’s referent.</p>

  2385. <p>If <code class="docutils literal notranslate"><span class="pre">proxy</span></code> is a proxy whose referent is <code class="docutils literal notranslate"><span class="pre">obj</span></code> then the expression</p>

  2386. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">proxy</span><span class="o">.</span><span class="n">_callmethod</span><span class="p">(</span><span class="n">methodname</span><span class="p">,</span> <span class="n">args</span><span class="p">,</span> <span class="n">kwds</span><span class="p">)</span>

  2387. </pre></div>

  2388. </div>

  2389. <p>will evaluate the expression</p>

  2390. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">getattr</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">methodname</span><span class="p">)(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">)</span>

  2391. </pre></div>

  2392. </div>

  2393. <p>in the manager’s process.</p>

  2394. <p>The returned value will be a copy of the result of the call or a proxy to

  2395. a new shared object – see documentation for the <em>method_to_typeid</em>

  2396. argument of <a class="reference internal" href="#multiprocessing.managers.BaseManager.register" title="multiprocessing.managers.BaseManager.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">BaseManager.register()</span></code></a>.</p>

  2397. <p>If an exception is raised by the call, then is re-raised by

  2398. <a class="reference internal" href="#multiprocessing.managers.BaseProxy._callmethod" title="multiprocessing.managers.BaseProxy._callmethod"><code class="xref py py-meth docutils literal notranslate"><span class="pre">_callmethod()</span></code></a>.  If some other exception is raised in the manager’s

  2399. process then this is converted into a <code class="xref py py-exc docutils literal notranslate"><span class="pre">RemoteError</span></code> exception and is

  2400. raised by <a class="reference internal" href="#multiprocessing.managers.BaseProxy._callmethod" title="multiprocessing.managers.BaseProxy._callmethod"><code class="xref py py-meth docutils literal notranslate"><span class="pre">_callmethod()</span></code></a>.</p>

  2401. <p>Note in particular that an exception will be raised if <em>methodname</em> has

  2402. not been <em>exposed</em>.</p>

  2403. <p>An example of the usage of <a class="reference internal" href="#multiprocessing.managers.BaseProxy._callmethod" title="multiprocessing.managers.BaseProxy._callmethod"><code class="xref py py-meth docutils literal notranslate"><span class="pre">_callmethod()</span></code></a>:</p>

  2404. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">l</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>

  2405. <span class="gp">&gt;&gt;&gt; </span><span class="n">l</span><span class="o">.</span><span class="n">_callmethod</span><span class="p">(</span><span class="s1">&#39;__len__&#39;</span><span class="p">)</span>

  2406. <span class="go">10</span>

  2407. <span class="gp">&gt;&gt;&gt; </span><span class="n">l</span><span class="o">.</span><span class="n">_callmethod</span><span class="p">(</span><span class="s1">&#39;__getitem__&#39;</span><span class="p">,</span> <span class="p">(</span><span class="nb">slice</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">7</span><span class="p">),))</span> <span class="c1"># equivalent to l[2:7]</span>

  2408. <span class="go">[2, 3, 4, 5, 6]</span>

  2409. <span class="gp">&gt;&gt;&gt; </span><span class="n">l</span><span class="o">.</span><span class="n">_callmethod</span><span class="p">(</span><span class="s1">&#39;__getitem__&#39;</span><span class="p">,</span> <span class="p">(</span><span class="mi">20</span><span class="p">,))</span>          <span class="c1"># equivalent to l[20]</span>

  2410. <span class="gt">Traceback (most recent call last):</span>

  2411. <span class="c">...</span>

  2412. <span class="gr">IndexError</span>: <span class="n">list index out of range</span>

  2413. </pre></div>

  2414. </div>

  2415. </dd></dl>

  2416. 

  2417. <dl class="py method">

  2418. <dt class="sig sig-object py" id="multiprocessing.managers.BaseProxy._getvalue">

  2419. <span class="sig-name descname"><span class="pre">_getvalue</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseProxy._getvalue" title="Link to this definition">¶</a></dt>

  2420. <dd><p>Return a copy of the referent.</p>

  2421. <p>If the referent is unpicklable then this will raise an exception.</p>

  2422. </dd></dl>

  2423. 

  2424. <dl class="py method">

  2425. <dt class="sig sig-object py" id="multiprocessing.managers.BaseProxy.__repr__">

  2426. <span class="sig-name descname"><span class="pre">__repr__</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseProxy.__repr__" title="Link to this definition">¶</a></dt>

  2427. <dd><p>Return a representation of the proxy object.</p>

  2428. </dd></dl>

  2429. 

  2430. <dl class="py method">

  2431. <dt class="sig sig-object py" id="multiprocessing.managers.BaseProxy.__str__">

  2432. <span class="sig-name descname"><span class="pre">__str__</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.managers.BaseProxy.__str__" title="Link to this definition">¶</a></dt>

  2433. <dd><p>Return the representation of the referent.</p>

  2434. </dd></dl>

  2435. 

  2436. </dd></dl>

  2437. 

  2438. <section id="cleanup">

  2439. <h4>Cleanup<a class="headerlink" href="#cleanup" title="Link to this heading">¶</a></h4>

  2440. <p>A proxy object uses a weakref callback so that when it gets garbage collected it

  2441. deregisters itself from the manager which owns its referent.</p>

  2442. <p>A shared object gets deleted from the manager process when there are no longer

  2443. any proxies referring to it.</p>

  2444. </section>

  2445. </section>

  2446. <section id="module-multiprocessing.pool">

  2447. <span id="process-pools"></span><h3>Process Pools<a class="headerlink" href="#module-multiprocessing.pool" title="Link to this heading">¶</a></h3>

  2448. <p>One can create a pool of processes which will carry out tasks submitted to it

  2449. with the <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a> class.</p>

  2450. <dl class="py class">

  2451. <dt class="sig sig-object py" id="multiprocessing.pool.Pool">

  2452. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.pool.</span></span><span class="sig-name descname"><span class="pre">Pool</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">processes</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">initializer</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">initargs</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">maxtasksperchild</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">context</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool" title="Link to this definition">¶</a></dt>

  2453. <dd><p>A process pool object which controls a pool of worker processes to which jobs

  2454. can be submitted.  It supports asynchronous results with timeouts and

  2455. callbacks and has a parallel map implementation.</p>

  2456. <p><em>processes</em> is the number of worker processes to use.  If <em>processes</em> is

  2457. <code class="docutils literal notranslate"><span class="pre">None</span></code> then the number returned by <a class="reference internal" href="os.html#os.cpu_count" title="os.cpu_count"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.cpu_count()</span></code></a> is used.</p>

  2458. <p>If <em>initializer</em> is not <code class="docutils literal notranslate"><span class="pre">None</span></code> then each worker process will call

  2459. <code class="docutils literal notranslate"><span class="pre">initializer(*initargs)</span></code> when it starts.</p>

  2460. <p><em>maxtasksperchild</em> is the number of tasks a worker process can complete

  2461. before it will exit and be replaced with a fresh worker process, to enable

  2462. unused resources to be freed. The default <em>maxtasksperchild</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code>, which

  2463. means worker processes will live as long as the pool.</p>

  2464. <p><em>context</em> can be used to specify the context used for starting

  2465. the worker processes.  Usually a pool is created using the

  2466. function <code class="xref py py-func docutils literal notranslate"><span class="pre">multiprocessing.Pool()</span></code> or the <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Pool()</span></code></a> method

  2467. of a context object.  In both cases <em>context</em> is set

  2468. appropriately.</p>

  2469. <p>Note that the methods of the pool object should only be called by

  2470. the process which created the pool.</p>

  2471. <div class="admonition warning">

  2472. <p class="admonition-title">Warning</p>

  2473. <p><a class="reference internal" href="#module-multiprocessing.pool" title="multiprocessing.pool: Create pools of processes."><code class="xref py py-class docutils literal notranslate"><span class="pre">multiprocessing.pool</span></code></a> objects have internal resources that need to be

  2474. properly managed (like any other resource) by using the pool as a context manager

  2475. or by calling <a class="reference internal" href="#multiprocessing.pool.Pool.close" title="multiprocessing.pool.Pool.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> and <a class="reference internal" href="#multiprocessing.pool.Pool.terminate" title="multiprocessing.pool.Pool.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">terminate()</span></code></a> manually. Failure to do this

  2476. can lead to the process hanging on finalization.</p>

  2477. <p>Note that it is <strong>not correct</strong> to rely on the garbage collector to destroy the pool

  2478. as CPython does not assure that the finalizer of the pool will be called

  2479. (see <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__del__()</span></code></a> for more information).</p>

  2480. </div>

  2481. <div class="versionchanged">

  2482. <p><span class="versionmodified changed">Changed in version 3.2: </span>Added the <em>maxtasksperchild</em> parameter.</p>

  2483. </div>

  2484. <div class="versionchanged">

  2485. <p><span class="versionmodified changed">Changed in version 3.4: </span>Added the <em>context</em> parameter.</p>

  2486. </div>

  2487. <div class="admonition note">

  2488. <p class="admonition-title">Note</p>

  2489. <p>Worker processes within a <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a> typically live for the complete

  2490. duration of the Pool’s work queue. A frequent pattern found in other

  2491. systems (such as Apache, mod_wsgi, etc) to free resources held by

  2492. workers is to allow a worker within a pool to complete only a set

  2493. amount of work before being exiting, being cleaned up and a new

  2494. process spawned to replace the old one. The <em>maxtasksperchild</em>

  2495. argument to the <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a> exposes this ability to the end user.</p>

  2496. </div>

  2497. <dl class="py method">

  2498. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.apply">

  2499. <span class="sig-name descname"><span class="pre">apply</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">func</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">args</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">kwds</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.apply" title="Link to this definition">¶</a></dt>

  2500. <dd><p>Call <em>func</em> with arguments <em>args</em> and keyword arguments <em>kwds</em>.  It blocks

  2501. until the result is ready. Given this blocks, <a class="reference internal" href="#multiprocessing.pool.Pool.apply_async" title="multiprocessing.pool.Pool.apply_async"><code class="xref py py-meth docutils literal notranslate"><span class="pre">apply_async()</span></code></a> is

  2502. better suited for performing work in parallel. Additionally, <em>func</em>

  2503. is only executed in one of the workers of the pool.</p>

  2504. </dd></dl>

  2505. 

  2506. <dl class="py method">

  2507. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.apply_async">

  2508. <span class="sig-name descname"><span class="pre">apply_async</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">func</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">args</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">kwds</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">callback</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">error_callback</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.apply_async" title="Link to this definition">¶</a></dt>

  2509. <dd><p>A variant of the <a class="reference internal" href="#multiprocessing.pool.Pool.apply" title="multiprocessing.pool.Pool.apply"><code class="xref py py-meth docutils literal notranslate"><span class="pre">apply()</span></code></a> method which returns a

  2510. <a class="reference internal" href="#multiprocessing.pool.AsyncResult" title="multiprocessing.pool.AsyncResult"><code class="xref py py-class docutils literal notranslate"><span class="pre">AsyncResult</span></code></a> object.</p>

  2511. <p>If <em>callback</em> is specified then it should be a callable which accepts a

  2512. single argument.  When the result becomes ready <em>callback</em> is applied to

  2513. it, that is unless the call failed, in which case the <em>error_callback</em>

  2514. is applied instead.</p>

  2515. <p>If <em>error_callback</em> is specified then it should be a callable which

  2516. accepts a single argument.  If the target function fails, then

  2517. the <em>error_callback</em> is called with the exception instance.</p>

  2518. <p>Callbacks should complete immediately since otherwise the thread which

  2519. handles the results will get blocked.</p>

  2520. </dd></dl>

  2521. 

  2522. <dl class="py method">

  2523. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.map">

  2524. <span class="sig-name descname"><span class="pre">map</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">func</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">iterable</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">chunksize</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.map" title="Link to this definition">¶</a></dt>

  2525. <dd><p>A parallel equivalent of the <a class="reference internal" href="functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate"><span class="pre">map()</span></code></a> built-in function (it supports only

  2526. one <em>iterable</em> argument though, for multiple iterables see <a class="reference internal" href="#multiprocessing.pool.Pool.starmap" title="multiprocessing.pool.Pool.starmap"><code class="xref py py-meth docutils literal notranslate"><span class="pre">starmap()</span></code></a>).

  2527. It blocks until the result is ready.</p>

  2528. <p>This method chops the iterable into a number of chunks which it submits to

  2529. the process pool as separate tasks.  The (approximate) size of these

  2530. chunks can be specified by setting <em>chunksize</em> to a positive integer.</p>

  2531. <p>Note that it may cause high memory usage for very long iterables. Consider

  2532. using <a class="reference internal" href="#multiprocessing.pool.Pool.imap" title="multiprocessing.pool.Pool.imap"><code class="xref py py-meth docutils literal notranslate"><span class="pre">imap()</span></code></a> or <a class="reference internal" href="#multiprocessing.pool.Pool.imap_unordered" title="multiprocessing.pool.Pool.imap_unordered"><code class="xref py py-meth docutils literal notranslate"><span class="pre">imap_unordered()</span></code></a> with explicit <em>chunksize</em>

  2533. option for better efficiency.</p>

  2534. </dd></dl>

  2535. 

  2536. <dl class="py method">

  2537. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.map_async">

  2538. <span class="sig-name descname"><span class="pre">map_async</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">func</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">iterable</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">chunksize</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">callback</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">error_callback</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.map_async" title="Link to this definition">¶</a></dt>

  2539. <dd><p>A variant of the <a class="reference internal" href="#multiprocessing.pool.Pool.map" title="multiprocessing.pool.Pool.map"><code class="xref py py-meth docutils literal notranslate"><span class="pre">map()</span></code></a> method which returns a

  2540. <a class="reference internal" href="#multiprocessing.pool.AsyncResult" title="multiprocessing.pool.AsyncResult"><code class="xref py py-class docutils literal notranslate"><span class="pre">AsyncResult</span></code></a> object.</p>

  2541. <p>If <em>callback</em> is specified then it should be a callable which accepts a

  2542. single argument.  When the result becomes ready <em>callback</em> is applied to

  2543. it, that is unless the call failed, in which case the <em>error_callback</em>

  2544. is applied instead.</p>

  2545. <p>If <em>error_callback</em> is specified then it should be a callable which

  2546. accepts a single argument.  If the target function fails, then

  2547. the <em>error_callback</em> is called with the exception instance.</p>

  2548. <p>Callbacks should complete immediately since otherwise the thread which

  2549. handles the results will get blocked.</p>

  2550. </dd></dl>

  2551. 

  2552. <dl class="py method">

  2553. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.imap">

  2554. <span class="sig-name descname"><span class="pre">imap</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">func</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">iterable</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">chunksize</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.imap" title="Link to this definition">¶</a></dt>

  2555. <dd><p>A lazier version of <a class="reference internal" href="#multiprocessing.pool.Pool.map" title="multiprocessing.pool.Pool.map"><code class="xref py py-meth docutils literal notranslate"><span class="pre">map()</span></code></a>.</p>

  2556. <p>The <em>chunksize</em> argument is the same as the one used by the <a class="reference internal" href="#multiprocessing.pool.Pool.map" title="multiprocessing.pool.Pool.map"><code class="xref py py-meth docutils literal notranslate"><span class="pre">map()</span></code></a>

  2557. method.  For very long iterables using a large value for <em>chunksize</em> can

  2558. make the job complete <strong>much</strong> faster than using the default value of

  2559. <code class="docutils literal notranslate"><span class="pre">1</span></code>.</p>

  2560. <p>Also if <em>chunksize</em> is <code class="docutils literal notranslate"><span class="pre">1</span></code> then the <code class="xref py py-meth docutils literal notranslate"><span class="pre">next()</span></code> method of the iterator

  2561. returned by the <a class="reference internal" href="#multiprocessing.pool.Pool.imap" title="multiprocessing.pool.Pool.imap"><code class="xref py py-meth docutils literal notranslate"><span class="pre">imap()</span></code></a> method has an optional <em>timeout</em> parameter:

  2562. <code class="docutils literal notranslate"><span class="pre">next(timeout)</span></code> will raise <a class="reference internal" href="#multiprocessing.TimeoutError" title="multiprocessing.TimeoutError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">multiprocessing.TimeoutError</span></code></a> if the

  2563. result cannot be returned within <em>timeout</em> seconds.</p>

  2564. </dd></dl>

  2565. 

  2566. <dl class="py method">

  2567. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.imap_unordered">

  2568. <span class="sig-name descname"><span class="pre">imap_unordered</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">func</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">iterable</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">chunksize</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.imap_unordered" title="Link to this definition">¶</a></dt>

  2569. <dd><p>The same as <a class="reference internal" href="#multiprocessing.pool.Pool.imap" title="multiprocessing.pool.Pool.imap"><code class="xref py py-meth docutils literal notranslate"><span class="pre">imap()</span></code></a> except that the ordering of the results from the

  2570. returned iterator should be considered arbitrary.  (Only when there is

  2571. only one worker process is the order guaranteed to be “correct”.)</p>

  2572. </dd></dl>

  2573. 

  2574. <dl class="py method">

  2575. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.starmap">

  2576. <span class="sig-name descname"><span class="pre">starmap</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">func</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">iterable</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">chunksize</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.starmap" title="Link to this definition">¶</a></dt>

  2577. <dd><p>Like <a class="reference internal" href="#multiprocessing.pool.Pool.map" title="multiprocessing.pool.Pool.map"><code class="xref py py-meth docutils literal notranslate"><span class="pre">map()</span></code></a> except that the

  2578. elements of the <em>iterable</em> are expected to be iterables that are

  2579. unpacked as arguments.</p>

  2580. <p>Hence an <em>iterable</em> of <code class="docutils literal notranslate"><span class="pre">[(1,2),</span> <span class="pre">(3,</span> <span class="pre">4)]</span></code> results in <code class="docutils literal notranslate"><span class="pre">[func(1,2),</span>

  2581. <span class="pre">func(3,4)]</span></code>.</p>

  2582. <div class="versionadded">

  2583. <p><span class="versionmodified added">New in version 3.3.</span></p>

  2584. </div>

  2585. </dd></dl>

  2586. 

  2587. <dl class="py method">

  2588. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.starmap_async">

  2589. <span class="sig-name descname"><span class="pre">starmap_async</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">func</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">iterable</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">chunksize</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">callback</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">error_callback</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.starmap_async" title="Link to this definition">¶</a></dt>

  2590. <dd><p>A combination of <a class="reference internal" href="#multiprocessing.pool.Pool.starmap" title="multiprocessing.pool.Pool.starmap"><code class="xref py py-meth docutils literal notranslate"><span class="pre">starmap()</span></code></a> and <a class="reference internal" href="#multiprocessing.pool.Pool.map_async" title="multiprocessing.pool.Pool.map_async"><code class="xref py py-meth docutils literal notranslate"><span class="pre">map_async()</span></code></a> that iterates over

  2591. <em>iterable</em> of iterables and calls <em>func</em> with the iterables unpacked.

  2592. Returns a result object.</p>

  2593. <div class="versionadded">

  2594. <p><span class="versionmodified added">New in version 3.3.</span></p>

  2595. </div>

  2596. </dd></dl>

  2597. 

  2598. <dl class="py method">

  2599. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.close">

  2600. <span class="sig-name descname"><span class="pre">close</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.close" title="Link to this definition">¶</a></dt>

  2601. <dd><p>Prevents any more tasks from being submitted to the pool.  Once all the

  2602. tasks have been completed the worker processes will exit.</p>

  2603. </dd></dl>

  2604. 

  2605. <dl class="py method">

  2606. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.terminate">

  2607. <span class="sig-name descname"><span class="pre">terminate</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.terminate" title="Link to this definition">¶</a></dt>

  2608. <dd><p>Stops the worker processes immediately without completing outstanding

  2609. work.  When the pool object is garbage collected <a class="reference internal" href="#multiprocessing.pool.Pool.terminate" title="multiprocessing.pool.Pool.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">terminate()</span></code></a> will be

  2610. called immediately.</p>

  2611. </dd></dl>

  2612. 

  2613. <dl class="py method">

  2614. <dt class="sig sig-object py" id="multiprocessing.pool.Pool.join">

  2615. <span class="sig-name descname"><span class="pre">join</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.Pool.join" title="Link to this definition">¶</a></dt>

  2616. <dd><p>Wait for the worker processes to exit.  One must call <a class="reference internal" href="#multiprocessing.pool.Pool.close" title="multiprocessing.pool.Pool.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> or

  2617. <a class="reference internal" href="#multiprocessing.pool.Pool.terminate" title="multiprocessing.pool.Pool.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">terminate()</span></code></a> before using <a class="reference internal" href="#multiprocessing.pool.Pool.join" title="multiprocessing.pool.Pool.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a>.</p>

  2618. </dd></dl>

  2619. 

  2620. <div class="versionchanged">

  2621. <p><span class="versionmodified changed">Changed in version 3.3: </span>Pool objects now support the context management protocol – see

  2622. <a class="reference internal" href="stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a>.  <a class="reference internal" href="stdtypes.html#contextmanager.__enter__" title="contextmanager.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> returns the

  2623. pool object, and <a class="reference internal" href="stdtypes.html#contextmanager.__exit__" title="contextmanager.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> calls <a class="reference internal" href="#multiprocessing.pool.Pool.terminate" title="multiprocessing.pool.Pool.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">terminate()</span></code></a>.</p>

  2624. </div>

  2625. </dd></dl>

  2626. 

  2627. <dl class="py class">

  2628. <dt class="sig sig-object py" id="multiprocessing.pool.AsyncResult">

  2629. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.pool.</span></span><span class="sig-name descname"><span class="pre">AsyncResult</span></span><a class="headerlink" href="#multiprocessing.pool.AsyncResult" title="Link to this definition">¶</a></dt>

  2630. <dd><p>The class of the result returned by <a class="reference internal" href="#multiprocessing.pool.Pool.apply_async" title="multiprocessing.pool.Pool.apply_async"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Pool.apply_async()</span></code></a> and

  2631. <a class="reference internal" href="#multiprocessing.pool.Pool.map_async" title="multiprocessing.pool.Pool.map_async"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Pool.map_async()</span></code></a>.</p>

  2632. <dl class="py method">

  2633. <dt class="sig sig-object py" id="multiprocessing.pool.AsyncResult.get">

  2634. <span class="sig-name descname"><span class="pre">get</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">timeout</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.AsyncResult.get" title="Link to this definition">¶</a></dt>

  2635. <dd><p>Return the result when it arrives.  If <em>timeout</em> is not <code class="docutils literal notranslate"><span class="pre">None</span></code> and the

  2636. result does not arrive within <em>timeout</em> seconds then

  2637. <a class="reference internal" href="#multiprocessing.TimeoutError" title="multiprocessing.TimeoutError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">multiprocessing.TimeoutError</span></code></a> is raised.  If the remote call raised

  2638. an exception then that exception will be reraised by <a class="reference internal" href="#multiprocessing.pool.AsyncResult.get" title="multiprocessing.pool.AsyncResult.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code></a>.</p>

  2639. </dd></dl>

  2640. 

  2641. <dl class="py method">

  2642. <dt class="sig sig-object py" id="multiprocessing.pool.AsyncResult.wait">

  2643. <span class="sig-name descname"><span class="pre">wait</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">timeout</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.AsyncResult.wait" title="Link to this definition">¶</a></dt>

  2644. <dd><p>Wait until the result is available or until <em>timeout</em> seconds pass.</p>

  2645. </dd></dl>

  2646. 

  2647. <dl class="py method">

  2648. <dt class="sig sig-object py" id="multiprocessing.pool.AsyncResult.ready">

  2649. <span class="sig-name descname"><span class="pre">ready</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.AsyncResult.ready" title="Link to this definition">¶</a></dt>

  2650. <dd><p>Return whether the call has completed.</p>

  2651. </dd></dl>

  2652. 

  2653. <dl class="py method">

  2654. <dt class="sig sig-object py" id="multiprocessing.pool.AsyncResult.successful">

  2655. <span class="sig-name descname"><span class="pre">successful</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.AsyncResult.successful" title="Link to this definition">¶</a></dt>

  2656. <dd><p>Return whether the call completed without raising an exception.  Will

  2657. raise <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> if the result is not ready.</p>

  2658. <div class="versionchanged">

  2659. <p><span class="versionmodified changed">Changed in version 3.7: </span>If the result is not ready, <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised instead of

  2660. <a class="reference internal" href="exceptions.html#AssertionError" title="AssertionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AssertionError</span></code></a>.</p>

  2661. </div>

  2662. </dd></dl>

  2663. 

  2664. </dd></dl>

  2665. 

  2666. <p>The following example demonstrates the use of a pool:</p>

  2667. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Pool</span>

  2668. <span class="kn">import</span> <span class="nn">time</span>

  2669. 

  2670. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>

  2671.     <span class="k">return</span> <span class="n">x</span><span class="o">*</span><span class="n">x</span>

  2672. 

  2673. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  2674.     <span class="k">with</span> <span class="n">Pool</span><span class="p">(</span><span class="n">processes</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span> <span class="k">as</span> <span class="n">pool</span><span class="p">:</span>         <span class="c1"># start 4 worker processes</span>

  2675.         <span class="n">result</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">apply_async</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="p">(</span><span class="mi">10</span><span class="p">,))</span> <span class="c1"># evaluate &quot;f(10)&quot; asynchronously in a single process</span>

  2676.         <span class="nb">print</span><span class="p">(</span><span class="n">result</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">1</span><span class="p">))</span>        <span class="c1"># prints &quot;100&quot; unless your computer is *very* slow</span>

  2677. 

  2678.         <span class="nb">print</span><span class="p">(</span><span class="n">pool</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)))</span>       <span class="c1"># prints &quot;[0, 1, 4,..., 81]&quot;</span>

  2679. 

  2680.         <span class="n">it</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">imap</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>

  2681.         <span class="nb">print</span><span class="p">(</span><span class="nb">next</span><span class="p">(</span><span class="n">it</span><span class="p">))</span>                     <span class="c1"># prints &quot;0&quot;</span>

  2682.         <span class="nb">print</span><span class="p">(</span><span class="nb">next</span><span class="p">(</span><span class="n">it</span><span class="p">))</span>                     <span class="c1"># prints &quot;1&quot;</span>

  2683.         <span class="nb">print</span><span class="p">(</span><span class="n">it</span><span class="o">.</span><span class="n">next</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">1</span><span class="p">))</span>           <span class="c1"># prints &quot;4&quot; unless your computer is *very* slow</span>

  2684. 

  2685.         <span class="n">result</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">apply_async</span><span class="p">(</span><span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">,</span> <span class="p">(</span><span class="mi">10</span><span class="p">,))</span>

  2686.         <span class="nb">print</span><span class="p">(</span><span class="n">result</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">1</span><span class="p">))</span>        <span class="c1"># raises multiprocessing.TimeoutError</span>

  2687. </pre></div>

  2688. </div>

  2689. </section>

  2690. <section id="module-multiprocessing.connection">

  2691. <span id="listeners-and-clients"></span><span id="multiprocessing-listeners-clients"></span><h3>Listeners and Clients<a class="headerlink" href="#module-multiprocessing.connection" title="Link to this heading">¶</a></h3>

  2692. <p>Usually message passing between processes is done using queues or by using

  2693. <a class="reference internal" href="#multiprocessing.connection.Connection" title="multiprocessing.connection.Connection"><code class="xref py py-class docutils literal notranslate"><span class="pre">Connection</span></code></a> objects returned by

  2694. <a class="reference internal" href="#multiprocessing.Pipe" title="multiprocessing.Pipe"><code class="xref py py-func docutils literal notranslate"><span class="pre">Pipe()</span></code></a>.</p>

  2695. <p>However, the <a class="reference internal" href="#module-multiprocessing.connection" title="multiprocessing.connection: API for dealing with sockets."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.connection</span></code></a> module allows some extra

  2696. flexibility.  It basically gives a high level message oriented API for dealing

  2697. with sockets or Windows named pipes.  It also has support for <em>digest

  2698. authentication</em> using the <a class="reference internal" href="hmac.html#module-hmac" title="hmac: Keyed-Hashing for Message Authentication (HMAC) implementation"><code class="xref py py-mod docutils literal notranslate"><span class="pre">hmac</span></code></a> module, and for polling

  2699. multiple connections at the same time.</p>

  2700. <dl class="py function">

  2701. <dt class="sig sig-object py" id="multiprocessing.connection.deliver_challenge">

  2702. <span class="sig-prename descclassname"><span class="pre">multiprocessing.connection.</span></span><span class="sig-name descname"><span class="pre">deliver_challenge</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">connection</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">authkey</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.deliver_challenge" title="Link to this definition">¶</a></dt>

  2703. <dd><p>Send a randomly generated message to the other end of the connection and wait

  2704. for a reply.</p>

  2705. <p>If the reply matches the digest of the message using <em>authkey</em> as the key

  2706. then a welcome message is sent to the other end of the connection.  Otherwise

  2707. <a class="reference internal" href="#multiprocessing.AuthenticationError" title="multiprocessing.AuthenticationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AuthenticationError</span></code></a> is raised.</p>

  2708. </dd></dl>

  2709. 

  2710. <dl class="py function">

  2711. <dt class="sig sig-object py" id="multiprocessing.connection.answer_challenge">

  2712. <span class="sig-prename descclassname"><span class="pre">multiprocessing.connection.</span></span><span class="sig-name descname"><span class="pre">answer_challenge</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">connection</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">authkey</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.answer_challenge" title="Link to this definition">¶</a></dt>

  2713. <dd><p>Receive a message, calculate the digest of the message using <em>authkey</em> as the

  2714. key, and then send the digest back.</p>

  2715. <p>If a welcome message is not received, then

  2716. <a class="reference internal" href="#multiprocessing.AuthenticationError" title="multiprocessing.AuthenticationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AuthenticationError</span></code></a> is raised.</p>

  2717. </dd></dl>

  2718. 

  2719. <dl class="py function">

  2720. <dt class="sig sig-object py" id="multiprocessing.connection.Client">

  2721. <span class="sig-prename descclassname"><span class="pre">multiprocessing.connection.</span></span><span class="sig-name descname"><span class="pre">Client</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">address</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">family</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">authkey</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Client" title="Link to this definition">¶</a></dt>

  2722. <dd><p>Attempt to set up a connection to the listener which is using address

  2723. <em>address</em>, returning a <a class="reference internal" href="#multiprocessing.connection.Connection" title="multiprocessing.connection.Connection"><code class="xref py py-class docutils literal notranslate"><span class="pre">Connection</span></code></a>.</p>

  2724. <p>The type of the connection is determined by <em>family</em> argument, but this can

  2725. generally be omitted since it can usually be inferred from the format of

  2726. <em>address</em>. (See <a class="reference internal" href="#multiprocessing-address-formats"><span class="std std-ref">Address Formats</span></a>)</p>

  2727. <p>If <em>authkey</em> is given and not None, it should be a byte string and will be

  2728. used as the secret key for an HMAC-based authentication challenge. No

  2729. authentication is done if <em>authkey</em> is None.

  2730. <a class="reference internal" href="#multiprocessing.AuthenticationError" title="multiprocessing.AuthenticationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AuthenticationError</span></code></a> is raised if authentication fails.

  2731. See <a class="reference internal" href="#multiprocessing-auth-keys"><span class="std std-ref">Authentication keys</span></a>.</p>

  2732. </dd></dl>

  2733. 

  2734. <dl class="py class">

  2735. <dt class="sig sig-object py" id="multiprocessing.connection.Listener">

  2736. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.connection.</span></span><span class="sig-name descname"><span class="pre">Listener</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">address</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">family</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">backlog</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">authkey</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Listener" title="Link to this definition">¶</a></dt>

  2737. <dd><p>A wrapper for a bound socket or Windows named pipe which is ‘listening’ for

  2738. connections.</p>

  2739. <p><em>address</em> is the address to be used by the bound socket or named pipe of the

  2740. listener object.</p>

  2741. <div class="admonition note">

  2742. <p class="admonition-title">Note</p>

  2743. <p>If an address of ‘0.0.0.0’ is used, the address will not be a connectable

  2744. end point on Windows. If you require a connectable end-point,

  2745. you should use ‘127.0.0.1’.</p>

  2746. </div>

  2747. <p><em>family</em> is the type of socket (or named pipe) to use.  This can be one of

  2748. the strings <code class="docutils literal notranslate"><span class="pre">'AF_INET'</span></code> (for a TCP socket), <code class="docutils literal notranslate"><span class="pre">'AF_UNIX'</span></code> (for a Unix

  2749. domain socket) or <code class="docutils literal notranslate"><span class="pre">'AF_PIPE'</span></code> (for a Windows named pipe).  Of these only

  2750. the first is guaranteed to be available.  If <em>family</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> then the

  2751. family is inferred from the format of <em>address</em>.  If <em>address</em> is also

  2752. <code class="docutils literal notranslate"><span class="pre">None</span></code> then a default is chosen.  This default is the family which is

  2753. assumed to be the fastest available.  See

  2754. <a class="reference internal" href="#multiprocessing-address-formats"><span class="std std-ref">Address Formats</span></a>.  Note that if <em>family</em> is

  2755. <code class="docutils literal notranslate"><span class="pre">'AF_UNIX'</span></code> and address is <code class="docutils literal notranslate"><span class="pre">None</span></code> then the socket will be created in a

  2756. private temporary directory created using <a class="reference internal" href="tempfile.html#tempfile.mkstemp" title="tempfile.mkstemp"><code class="xref py py-func docutils literal notranslate"><span class="pre">tempfile.mkstemp()</span></code></a>.</p>

  2757. <p>If the listener object uses a socket then <em>backlog</em> (1 by default) is passed

  2758. to the <a class="reference internal" href="socket.html#socket.socket.listen" title="socket.socket.listen"><code class="xref py py-meth docutils literal notranslate"><span class="pre">listen()</span></code></a> method of the socket once it has been

  2759. bound.</p>

  2760. <p>If <em>authkey</em> is given and not None, it should be a byte string and will be

  2761. used as the secret key for an HMAC-based authentication challenge. No

  2762. authentication is done if <em>authkey</em> is None.

  2763. <a class="reference internal" href="#multiprocessing.AuthenticationError" title="multiprocessing.AuthenticationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AuthenticationError</span></code></a> is raised if authentication fails.

  2764. See <a class="reference internal" href="#multiprocessing-auth-keys"><span class="std std-ref">Authentication keys</span></a>.</p>

  2765. <dl class="py method">

  2766. <dt class="sig sig-object py" id="multiprocessing.connection.Listener.accept">

  2767. <span class="sig-name descname"><span class="pre">accept</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Listener.accept" title="Link to this definition">¶</a></dt>

  2768. <dd><p>Accept a connection on the bound socket or named pipe of the listener

  2769. object and return a <a class="reference internal" href="#multiprocessing.connection.Connection" title="multiprocessing.connection.Connection"><code class="xref py py-class docutils literal notranslate"><span class="pre">Connection</span></code></a> object.

  2770. If authentication is attempted and fails, then

  2771. <a class="reference internal" href="#multiprocessing.AuthenticationError" title="multiprocessing.AuthenticationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AuthenticationError</span></code></a> is raised.</p>

  2772. </dd></dl>

  2773. 

  2774. <dl class="py method">

  2775. <dt class="sig sig-object py" id="multiprocessing.connection.Listener.close">

  2776. <span class="sig-name descname"><span class="pre">close</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.Listener.close" title="Link to this definition">¶</a></dt>

  2777. <dd><p>Close the bound socket or named pipe of the listener object.  This is

  2778. called automatically when the listener is garbage collected.  However it

  2779. is advisable to call it explicitly.</p>

  2780. </dd></dl>

  2781. 

  2782. <p>Listener objects have the following read-only properties:</p>

  2783. <dl class="py attribute">

  2784. <dt class="sig sig-object py" id="multiprocessing.connection.Listener.address">

  2785. <span class="sig-name descname"><span class="pre">address</span></span><a class="headerlink" href="#multiprocessing.connection.Listener.address" title="Link to this definition">¶</a></dt>

  2786. <dd><p>The address which is being used by the Listener object.</p>

  2787. </dd></dl>

  2788. 

  2789. <dl class="py attribute">

  2790. <dt class="sig sig-object py" id="multiprocessing.connection.Listener.last_accepted">

  2791. <span class="sig-name descname"><span class="pre">last_accepted</span></span><a class="headerlink" href="#multiprocessing.connection.Listener.last_accepted" title="Link to this definition">¶</a></dt>

  2792. <dd><p>The address from which the last accepted connection came.  If this is

  2793. unavailable then it is <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>

  2794. </dd></dl>

  2795. 

  2796. <div class="versionchanged">

  2797. <p><span class="versionmodified changed">Changed in version 3.3: </span>Listener objects now support the context management protocol – see

  2798. <a class="reference internal" href="stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a>.  <a class="reference internal" href="stdtypes.html#contextmanager.__enter__" title="contextmanager.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> returns the

  2799. listener object, and <a class="reference internal" href="stdtypes.html#contextmanager.__exit__" title="contextmanager.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> calls <a class="reference internal" href="#multiprocessing.connection.Listener.close" title="multiprocessing.connection.Listener.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a>.</p>

  2800. </div>

  2801. </dd></dl>

  2802. 

  2803. <dl class="py function">

  2804. <dt class="sig sig-object py" id="multiprocessing.connection.wait">

  2805. <span class="sig-prename descclassname"><span class="pre">multiprocessing.connection.</span></span><span class="sig-name descname"><span class="pre">wait</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">object_list</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.connection.wait" title="Link to this definition">¶</a></dt>

  2806. <dd><p>Wait till an object in <em>object_list</em> is ready.  Returns the list of

  2807. those objects in <em>object_list</em> which are ready.  If <em>timeout</em> is a

  2808. float then the call blocks for at most that many seconds.  If

  2809. <em>timeout</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> then it will block for an unlimited period.

  2810. A negative timeout is equivalent to a zero timeout.</p>

  2811. <p>For both POSIX and Windows, an object can appear in <em>object_list</em> if

  2812. it is</p>

  2813. <ul class="simple">

  2814. <li><p>a readable <a class="reference internal" href="#multiprocessing.connection.Connection" title="multiprocessing.connection.Connection"><code class="xref py py-class docutils literal notranslate"><span class="pre">Connection</span></code></a> object;</p></li>

  2815. <li><p>a connected and readable <a class="reference internal" href="socket.html#socket.socket" title="socket.socket"><code class="xref py py-class docutils literal notranslate"><span class="pre">socket.socket</span></code></a> object; or</p></li>

  2816. <li><p>the <a class="reference internal" href="#multiprocessing.Process.sentinel" title="multiprocessing.Process.sentinel"><code class="xref py py-attr docutils literal notranslate"><span class="pre">sentinel</span></code></a> attribute of a

  2817. <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> object.</p></li>

  2818. </ul>

  2819. <p>A connection or socket object is ready when there is data available

  2820. to be read from it, or the other end has been closed.</p>

  2821. <p><strong>POSIX</strong>: <code class="docutils literal notranslate"><span class="pre">wait(object_list,</span> <span class="pre">timeout)</span></code> almost equivalent

  2822. <code class="docutils literal notranslate"><span class="pre">select.select(object_list,</span> <span class="pre">[],</span> <span class="pre">[],</span> <span class="pre">timeout)</span></code>.  The difference is

  2823. that, if <a class="reference internal" href="select.html#select.select" title="select.select"><code class="xref py py-func docutils literal notranslate"><span class="pre">select.select()</span></code></a> is interrupted by a signal, it can

  2824. raise <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a> with an error number of <code class="docutils literal notranslate"><span class="pre">EINTR</span></code>, whereas

  2825. <a class="reference internal" href="#multiprocessing.connection.wait" title="multiprocessing.connection.wait"><code class="xref py py-func docutils literal notranslate"><span class="pre">wait()</span></code></a> will not.</p>

  2826. <p><strong>Windows</strong>: An item in <em>object_list</em> must either be an integer

  2827. handle which is waitable (according to the definition used by the

  2828. documentation of the Win32 function <code class="docutils literal notranslate"><span class="pre">WaitForMultipleObjects()</span></code>)

  2829. or it can be an object with a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> method which returns a

  2830. socket handle or pipe handle.  (Note that pipe handles and socket

  2831. handles are <strong>not</strong> waitable handles.)</p>

  2832. <div class="versionadded">

  2833. <p><span class="versionmodified added">New in version 3.3.</span></p>

  2834. </div>

  2835. </dd></dl>

  2836. 

  2837. <p><strong>Examples</strong></p>

  2838. <p>The following server code creates a listener which uses <code class="docutils literal notranslate"><span class="pre">'secret</span> <span class="pre">password'</span></code> as

  2839. an authentication key.  It then waits for a connection and sends some data to

  2840. the client:</p>

  2841. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing.connection</span> <span class="kn">import</span> <span class="n">Listener</span>

  2842. <span class="kn">from</span> <span class="nn">array</span> <span class="kn">import</span> <span class="n">array</span>

  2843. 

  2844. <span class="n">address</span> <span class="o">=</span> <span class="p">(</span><span class="s1">&#39;localhost&#39;</span><span class="p">,</span> <span class="mi">6000</span><span class="p">)</span>     <span class="c1"># family is deduced to be &#39;AF_INET&#39;</span>

  2845. 

  2846. <span class="k">with</span> <span class="n">Listener</span><span class="p">(</span><span class="n">address</span><span class="p">,</span> <span class="n">authkey</span><span class="o">=</span><span class="sa">b</span><span class="s1">&#39;secret password&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">listener</span><span class="p">:</span>

  2847.     <span class="k">with</span> <span class="n">listener</span><span class="o">.</span><span class="n">accept</span><span class="p">()</span> <span class="k">as</span> <span class="n">conn</span><span class="p">:</span>

  2848.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;connection accepted from&#39;</span><span class="p">,</span> <span class="n">listener</span><span class="o">.</span><span class="n">last_accepted</span><span class="p">)</span>

  2849. 

  2850.         <span class="n">conn</span><span class="o">.</span><span class="n">send</span><span class="p">([</span><span class="mf">2.25</span><span class="p">,</span> <span class="kc">None</span><span class="p">,</span> <span class="s1">&#39;junk&#39;</span><span class="p">,</span> <span class="nb">float</span><span class="p">])</span>

  2851. 

  2852.         <span class="n">conn</span><span class="o">.</span><span class="n">send_bytes</span><span class="p">(</span><span class="sa">b</span><span class="s1">&#39;hello&#39;</span><span class="p">)</span>

  2853. 

  2854.         <span class="n">conn</span><span class="o">.</span><span class="n">send_bytes</span><span class="p">(</span><span class="n">array</span><span class="p">(</span><span class="s1">&#39;i&#39;</span><span class="p">,</span> <span class="p">[</span><span class="mi">42</span><span class="p">,</span> <span class="mi">1729</span><span class="p">]))</span>

  2855. </pre></div>

  2856. </div>

  2857. <p>The following code connects to the server and receives some data from the

  2858. server:</p>

  2859. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing.connection</span> <span class="kn">import</span> <span class="n">Client</span>

  2860. <span class="kn">from</span> <span class="nn">array</span> <span class="kn">import</span> <span class="n">array</span>

  2861. 

  2862. <span class="n">address</span> <span class="o">=</span> <span class="p">(</span><span class="s1">&#39;localhost&#39;</span><span class="p">,</span> <span class="mi">6000</span><span class="p">)</span>

  2863. 

  2864. <span class="k">with</span> <span class="n">Client</span><span class="p">(</span><span class="n">address</span><span class="p">,</span> <span class="n">authkey</span><span class="o">=</span><span class="sa">b</span><span class="s1">&#39;secret password&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">conn</span><span class="p">:</span>

  2865.     <span class="nb">print</span><span class="p">(</span><span class="n">conn</span><span class="o">.</span><span class="n">recv</span><span class="p">())</span>                  <span class="c1"># =&gt; [2.25, None, &#39;junk&#39;, float]</span>

  2866. 

  2867.     <span class="nb">print</span><span class="p">(</span><span class="n">conn</span><span class="o">.</span><span class="n">recv_bytes</span><span class="p">())</span>            <span class="c1"># =&gt; &#39;hello&#39;</span>

  2868. 

  2869.     <span class="n">arr</span> <span class="o">=</span> <span class="n">array</span><span class="p">(</span><span class="s1">&#39;i&#39;</span><span class="p">,</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">])</span>

  2870.     <span class="nb">print</span><span class="p">(</span><span class="n">conn</span><span class="o">.</span><span class="n">recv_bytes_into</span><span class="p">(</span><span class="n">arr</span><span class="p">))</span>    <span class="c1"># =&gt; 8</span>

  2871.     <span class="nb">print</span><span class="p">(</span><span class="n">arr</span><span class="p">)</span>                          <span class="c1"># =&gt; array(&#39;i&#39;, [42, 1729, 0, 0, 0])</span>

  2872. </pre></div>

  2873. </div>

  2874. <p>The following code uses <a class="reference internal" href="#multiprocessing.connection.wait" title="multiprocessing.connection.wait"><code class="xref py py-func docutils literal notranslate"><span class="pre">wait()</span></code></a> to

  2875. wait for messages from multiple processes at once:</p>

  2876. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">time</span><span class="o">,</span> <span class="nn">random</span>

  2877. <span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Pipe</span><span class="p">,</span> <span class="n">current_process</span>

  2878. <span class="kn">from</span> <span class="nn">multiprocessing.connection</span> <span class="kn">import</span> <span class="n">wait</span>

  2879. 

  2880. <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">w</span><span class="p">):</span>

  2881.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>

  2882.         <span class="n">w</span><span class="o">.</span><span class="n">send</span><span class="p">((</span><span class="n">i</span><span class="p">,</span> <span class="n">current_process</span><span class="p">()</span><span class="o">.</span><span class="n">name</span><span class="p">))</span>

  2883.     <span class="n">w</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>

  2884. 

  2885. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  2886.     <span class="n">readers</span> <span class="o">=</span> <span class="p">[]</span>

  2887. 

  2888.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">):</span>

  2889.         <span class="n">r</span><span class="p">,</span> <span class="n">w</span> <span class="o">=</span> <span class="n">Pipe</span><span class="p">(</span><span class="n">duplex</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>

  2890.         <span class="n">readers</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">r</span><span class="p">)</span>

  2891.         <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">foo</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">w</span><span class="p">,))</span>

  2892.         <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  2893.         <span class="c1"># We close the writable end of the pipe now to be sure that</span>

  2894.         <span class="c1"># p is the only process which owns a handle for it.  This</span>

  2895.         <span class="c1"># ensures that when p closes its handle for the writable end,</span>

  2896.         <span class="c1"># wait() will promptly report the readable end as being ready.</span>

  2897.         <span class="n">w</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>

  2898. 

  2899.     <span class="k">while</span> <span class="n">readers</span><span class="p">:</span>

  2900.         <span class="k">for</span> <span class="n">r</span> <span class="ow">in</span> <span class="n">wait</span><span class="p">(</span><span class="n">readers</span><span class="p">):</span>

  2901.             <span class="k">try</span><span class="p">:</span>

  2902.                 <span class="n">msg</span> <span class="o">=</span> <span class="n">r</span><span class="o">.</span><span class="n">recv</span><span class="p">()</span>

  2903.             <span class="k">except</span> <span class="ne">EOFError</span><span class="p">:</span>

  2904.                 <span class="n">readers</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">r</span><span class="p">)</span>

  2905.             <span class="k">else</span><span class="p">:</span>

  2906.                 <span class="nb">print</span><span class="p">(</span><span class="n">msg</span><span class="p">)</span>

  2907. </pre></div>

  2908. </div>

  2909. <section id="address-formats">

  2910. <span id="multiprocessing-address-formats"></span><h4>Address Formats<a class="headerlink" href="#address-formats" title="Link to this heading">¶</a></h4>

  2911. <ul class="simple">

  2912. <li><p>An <code class="docutils literal notranslate"><span class="pre">'AF_INET'</span></code> address is a tuple of the form <code class="docutils literal notranslate"><span class="pre">(hostname,</span> <span class="pre">port)</span></code> where

  2913. <em>hostname</em> is a string and <em>port</em> is an integer.</p></li>

  2914. <li><p>An <code class="docutils literal notranslate"><span class="pre">'AF_UNIX'</span></code> address is a string representing a filename on the

  2915. filesystem.</p></li>

  2916. <li><p>An <code class="docutils literal notranslate"><span class="pre">'AF_PIPE'</span></code> address is a string of the form

  2917. <code class="samp docutils literal notranslate"><span class="pre">r'\\.\pipe\</span><em><span class="pre">PipeName</span></em><span class="pre">'</span></code>.  To use <a class="reference internal" href="#multiprocessing.connection.Client" title="multiprocessing.connection.Client"><code class="xref py py-func docutils literal notranslate"><span class="pre">Client()</span></code></a> to connect to a named

  2918. pipe on a remote computer called <em>ServerName</em> one should use an address of the

  2919. form <code class="samp docutils literal notranslate"><span class="pre">r'\\</span><em><span class="pre">ServerName</span></em><span class="pre">\pipe\</span><em><span class="pre">PipeName</span></em><span class="pre">'</span></code> instead.</p></li>

  2920. </ul>

  2921. <p>Note that any string beginning with two backslashes is assumed by default to be

  2922. an <code class="docutils literal notranslate"><span class="pre">'AF_PIPE'</span></code> address rather than an <code class="docutils literal notranslate"><span class="pre">'AF_UNIX'</span></code> address.</p>

  2923. </section>

  2924. </section>

  2925. <section id="authentication-keys">

  2926. <span id="multiprocessing-auth-keys"></span><h3>Authentication keys<a class="headerlink" href="#authentication-keys" title="Link to this heading">¶</a></h3>

  2927. <p>When one uses <a class="reference internal" href="#multiprocessing.connection.Connection.recv" title="multiprocessing.connection.Connection.recv"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Connection.recv</span></code></a>, the

  2928. data received is automatically

  2929. unpickled. Unfortunately unpickling data from an untrusted source is a security

  2930. risk. Therefore <a class="reference internal" href="#multiprocessing.connection.Listener" title="multiprocessing.connection.Listener"><code class="xref py py-class docutils literal notranslate"><span class="pre">Listener</span></code></a> and <a class="reference internal" href="#multiprocessing.connection.Client" title="multiprocessing.connection.Client"><code class="xref py py-func docutils literal notranslate"><span class="pre">Client()</span></code></a> use the <a class="reference internal" href="hmac.html#module-hmac" title="hmac: Keyed-Hashing for Message Authentication (HMAC) implementation"><code class="xref py py-mod docutils literal notranslate"><span class="pre">hmac</span></code></a> module

  2931. to provide digest authentication.</p>

  2932. <p>An authentication key is a byte string which can be thought of as a

  2933. password: once a connection is established both ends will demand proof

  2934. that the other knows the authentication key.  (Demonstrating that both

  2935. ends are using the same key does <strong>not</strong> involve sending the key over

  2936. the connection.)</p>

  2937. <p>If authentication is requested but no authentication key is specified then the

  2938. return value of <code class="docutils literal notranslate"><span class="pre">current_process().authkey</span></code> is used (see

  2939. <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a>).  This value will be automatically inherited by

  2940. any <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> object that the current process creates.

  2941. This means that (by default) all processes of a multi-process program will share

  2942. a single authentication key which can be used when setting up connections

  2943. between themselves.</p>

  2944. <p>Suitable authentication keys can also be generated by using <a class="reference internal" href="os.html#os.urandom" title="os.urandom"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.urandom()</span></code></a>.</p>

  2945. </section>

  2946. <section id="logging">

  2947. <h3>Logging<a class="headerlink" href="#logging" title="Link to this heading">¶</a></h3>

  2948. <p>Some support for logging is available.  Note, however, that the <a class="reference internal" href="logging.html#module-logging" title="logging: Flexible event logging system for applications."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging</span></code></a>

  2949. package does not use process shared locks so it is possible (depending on the

  2950. handler type) for messages from different processes to get mixed up.</p>

  2951. <dl class="py function">

  2952. <dt class="sig sig-object py" id="multiprocessing.get_logger">

  2953. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">get_logger</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.get_logger" title="Link to this definition">¶</a></dt>

  2954. <dd><p>Returns the logger used by <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a>.  If necessary, a new one

  2955. will be created.</p>

  2956. <p>When first created the logger has level <a class="reference internal" href="logging.html#logging.NOTSET" title="logging.NOTSET"><code class="xref py py-const docutils literal notranslate"><span class="pre">logging.NOTSET</span></code></a> and no

  2957. default handler. Messages sent to this logger will not by default propagate

  2958. to the root logger.</p>

  2959. <p>Note that on Windows child processes will only inherit the level of the

  2960. parent process’s logger – any other customization of the logger will not be

  2961. inherited.</p>

  2962. </dd></dl>

  2963. 

  2964. <dl class="py function">

  2965. <dt class="sig sig-object py" id="multiprocessing.log_to_stderr">

  2966. <span class="sig-prename descclassname"><span class="pre">multiprocessing.</span></span><span class="sig-name descname"><span class="pre">log_to_stderr</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">level</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.log_to_stderr" title="Link to this definition">¶</a></dt>

  2967. <dd><p>This function performs a call to <a class="reference internal" href="#multiprocessing.get_logger" title="multiprocessing.get_logger"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_logger()</span></code></a> but in addition to

  2968. returning the logger created by get_logger, it adds a handler which sends

  2969. output to <a class="reference internal" href="sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stderr</span></code></a> using format

  2970. <code class="docutils literal notranslate"><span class="pre">'[%(levelname)s/%(processName)s]</span> <span class="pre">%(message)s'</span></code>.

  2971. You can modify <code class="docutils literal notranslate"><span class="pre">levelname</span></code> of the logger by passing a <code class="docutils literal notranslate"><span class="pre">level</span></code> argument.</p>

  2972. </dd></dl>

  2973. 

  2974. <p>Below is an example session with logging turned on:</p>

  2975. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">multiprocessing</span><span class="o">,</span> <span class="nn">logging</span>

  2976. <span class="gp">&gt;&gt;&gt; </span><span class="n">logger</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">log_to_stderr</span><span class="p">()</span>

  2977. <span class="gp">&gt;&gt;&gt; </span><span class="n">logger</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">)</span>

  2978. <span class="gp">&gt;&gt;&gt; </span><span class="n">logger</span><span class="o">.</span><span class="n">warning</span><span class="p">(</span><span class="s1">&#39;doomed&#39;</span><span class="p">)</span>

  2979. <span class="go">[WARNING/MainProcess] doomed</span>

  2980. <span class="gp">&gt;&gt;&gt; </span><span class="n">m</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">Manager</span><span class="p">()</span>

  2981. <span class="go">[INFO/SyncManager-...] child process calling self.run()</span>

  2982. <span class="go">[INFO/SyncManager-...] created temp directory /.../pymp-...</span>

  2983. <span class="go">[INFO/SyncManager-...] manager serving at &#39;/.../listener-...&#39;</span>

  2984. <span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">m</span>

  2985. <span class="go">[INFO/MainProcess] sending shutdown message to manager</span>

  2986. <span class="go">[INFO/SyncManager-...] manager exiting with exitcode 0</span>

  2987. </pre></div>

  2988. </div>

  2989. <p>For a full table of logging levels, see the <a class="reference internal" href="logging.html#module-logging" title="logging: Flexible event logging system for applications."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging</span></code></a> module.</p>

  2990. </section>

  2991. <section id="module-multiprocessing.dummy">

  2992. <span id="the-multiprocessing-dummy-module"></span><h3>The <a class="reference internal" href="#module-multiprocessing.dummy" title="multiprocessing.dummy: Dumb wrapper around threading."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.dummy</span></code></a> module<a class="headerlink" href="#module-multiprocessing.dummy" title="Link to this heading">¶</a></h3>

  2993. <p><a class="reference internal" href="#module-multiprocessing.dummy" title="multiprocessing.dummy: Dumb wrapper around threading."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.dummy</span></code></a> replicates the API of <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> but is

  2994. no more than a wrapper around the <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module.</p>

  2995. <p>In particular, the <code class="docutils literal notranslate"><span class="pre">Pool</span></code> function provided by <a class="reference internal" href="#module-multiprocessing.dummy" title="multiprocessing.dummy: Dumb wrapper around threading."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.dummy</span></code></a>

  2996. returns an instance of <a class="reference internal" href="#multiprocessing.pool.ThreadPool" title="multiprocessing.pool.ThreadPool"><code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadPool</span></code></a>, which is a subclass of

  2997. <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a> that supports all the same method calls but uses a pool of

  2998. worker threads rather than worker processes.</p>

  2999. <dl class="py class">

  3000. <dt class="sig sig-object py" id="multiprocessing.pool.ThreadPool">

  3001. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">multiprocessing.pool.</span></span><span class="sig-name descname"><span class="pre">ThreadPool</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">processes</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">initializer</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">initargs</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#multiprocessing.pool.ThreadPool" title="Link to this definition">¶</a></dt>

  3002. <dd><p>A thread pool object which controls a pool of worker threads to which jobs

  3003. can be submitted.  <a class="reference internal" href="#multiprocessing.pool.ThreadPool" title="multiprocessing.pool.ThreadPool"><code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadPool</span></code></a> instances are fully interface

  3004. compatible with <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a> instances, and their resources must also be

  3005. properly managed, either by using the pool as a context manager or by

  3006. calling <a class="reference internal" href="#multiprocessing.pool.Pool.close" title="multiprocessing.pool.Pool.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> and

  3007. <a class="reference internal" href="#multiprocessing.pool.Pool.terminate" title="multiprocessing.pool.Pool.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">terminate()</span></code></a> manually.</p>

  3008. <p><em>processes</em> is the number of worker threads to use.  If <em>processes</em> is

  3009. <code class="docutils literal notranslate"><span class="pre">None</span></code> then the number returned by <a class="reference internal" href="os.html#os.cpu_count" title="os.cpu_count"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.cpu_count()</span></code></a> is used.</p>

  3010. <p>If <em>initializer</em> is not <code class="docutils literal notranslate"><span class="pre">None</span></code> then each worker process will call

  3011. <code class="docutils literal notranslate"><span class="pre">initializer(*initargs)</span></code> when it starts.</p>

  3012. <p>Unlike <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a>, <em>maxtasksperchild</em> and <em>context</em> cannot be provided.</p>

  3013. <div class="admonition note">

  3014. <p class="admonition-title">Note</p>

  3015. <p>A <a class="reference internal" href="#multiprocessing.pool.ThreadPool" title="multiprocessing.pool.ThreadPool"><code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadPool</span></code></a> shares the same interface as <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a>, which

  3016. is designed around a pool of processes and predates the introduction of

  3017. the <a class="reference internal" href="concurrent.futures.html#module-concurrent.futures" title="concurrent.futures: Execute computations concurrently using threads or processes."><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures</span></code></a> module.  As such, it inherits some

  3018. operations that don’t make sense for a pool backed by threads, and it

  3019. has its own type for representing the status of asynchronous jobs,

  3020. <a class="reference internal" href="#multiprocessing.pool.AsyncResult" title="multiprocessing.pool.AsyncResult"><code class="xref py py-class docutils literal notranslate"><span class="pre">AsyncResult</span></code></a>, that is not understood by any other libraries.</p>

  3021. <p>Users should generally prefer to use

  3022. <a class="reference internal" href="concurrent.futures.html#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures.ThreadPoolExecutor</span></code></a>, which has a simpler

  3023. interface that was designed around threads from the start, and which

  3024. returns <a class="reference internal" href="concurrent.futures.html#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures.Future</span></code></a> instances that are

  3025. compatible with many other libraries, including <a class="reference internal" href="asyncio.html#module-asyncio" title="asyncio: Asynchronous I/O."><code class="xref py py-mod docutils literal notranslate"><span class="pre">asyncio</span></code></a>.</p>

  3026. </div>

  3027. </dd></dl>

  3028. 

  3029. </section>

  3030. </section>

  3031. <section id="programming-guidelines">

  3032. <span id="multiprocessing-programming"></span><h2>Programming guidelines<a class="headerlink" href="#programming-guidelines" title="Link to this heading">¶</a></h2>

  3033. <p>There are certain guidelines and idioms which should be adhered to when using

  3034. <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a>.</p>

  3035. <section id="all-start-methods">

  3036. <h3>All start methods<a class="headerlink" href="#all-start-methods" title="Link to this heading">¶</a></h3>

  3037. <p>The following applies to all start methods.</p>

  3038. <p>Avoid shared state</p>

  3039. <blockquote>

  3040. <div><p>As far as possible one should try to avoid shifting large amounts of data

  3041. between processes.</p>

  3042. <p>It is probably best to stick to using queues or pipes for communication

  3043. between processes rather than using the lower level synchronization

  3044. primitives.</p>

  3045. </div></blockquote>

  3046. <p>Picklability</p>

  3047. <blockquote>

  3048. <div><p>Ensure that the arguments to the methods of proxies are picklable.</p>

  3049. </div></blockquote>

  3050. <p>Thread safety of proxies</p>

  3051. <blockquote>

  3052. <div><p>Do not use a proxy object from more than one thread unless you protect it

  3053. with a lock.</p>

  3054. <p>(There is never a problem with different processes using the <em>same</em> proxy.)</p>

  3055. </div></blockquote>

  3056. <p>Joining zombie processes</p>

  3057. <blockquote>

  3058. <div><p>On POSIX when a process finishes but has not been joined it becomes a zombie.

  3059. There should never be very many because each time a new process starts (or

  3060. <a class="reference internal" href="#multiprocessing.active_children" title="multiprocessing.active_children"><code class="xref py py-func docutils literal notranslate"><span class="pre">active_children()</span></code></a> is called) all completed processes

  3061. which have not yet been joined will be joined.  Also calling a finished

  3062. process’s <a class="reference internal" href="#multiprocessing.Process.is_alive" title="multiprocessing.Process.is_alive"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Process.is_alive</span></code></a> will

  3063. join the process.  Even so it is probably good

  3064. practice to explicitly join all the processes that you start.</p>

  3065. </div></blockquote>

  3066. <p>Better to inherit than pickle/unpickle</p>

  3067. <blockquote>

  3068. <div><p>When using the <em>spawn</em> or <em>forkserver</em> start methods many types

  3069. from <a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> need to be picklable so that child

  3070. processes can use them.  However, one should generally avoid

  3071. sending shared objects to other processes using pipes or queues.

  3072. Instead you should arrange the program so that a process which

  3073. needs access to a shared resource created elsewhere can inherit it

  3074. from an ancestor process.</p>

  3075. </div></blockquote>

  3076. <p>Avoid terminating processes</p>

  3077. <blockquote>

  3078. <div><p>Using the <a class="reference internal" href="#multiprocessing.Process.terminate" title="multiprocessing.Process.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Process.terminate</span></code></a>

  3079. method to stop a process is liable to

  3080. cause any shared resources (such as locks, semaphores, pipes and queues)

  3081. currently being used by the process to become broken or unavailable to other

  3082. processes.</p>

  3083. <p>Therefore it is probably best to only consider using

  3084. <a class="reference internal" href="#multiprocessing.Process.terminate" title="multiprocessing.Process.terminate"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Process.terminate</span></code></a> on processes

  3085. which never use any shared resources.</p>

  3086. </div></blockquote>

  3087. <p>Joining processes that use queues</p>

  3088. <blockquote>

  3089. <div><p>Bear in mind that a process that has put items in a queue will wait before

  3090. terminating until all the buffered items are fed by the “feeder” thread to

  3091. the underlying pipe.  (The child process can call the

  3092. <a class="reference internal" href="#multiprocessing.Queue.cancel_join_thread" title="multiprocessing.Queue.cancel_join_thread"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Queue.cancel_join_thread</span></code></a>

  3093. method of the queue to avoid this behaviour.)</p>

  3094. <p>This means that whenever you use a queue you need to make sure that all

  3095. items which have been put on the queue will eventually be removed before the

  3096. process is joined.  Otherwise you cannot be sure that processes which have

  3097. put items on the queue will terminate.  Remember also that non-daemonic

  3098. processes will be joined automatically.</p>

  3099. <p>An example which will deadlock is the following:</p>

  3100. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Queue</span>

  3101. 

  3102. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">q</span><span class="p">):</span>

  3103.     <span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="s1">&#39;X&#39;</span> <span class="o">*</span> <span class="mi">1000000</span><span class="p">)</span>

  3104. 

  3105. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  3106.     <span class="n">queue</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span>

  3107.     <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">queue</span><span class="p">,))</span>

  3108.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  3109.     <span class="n">p</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>                    <span class="c1"># this deadlocks</span>

  3110.     <span class="n">obj</span> <span class="o">=</span> <span class="n">queue</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>

  3111. </pre></div>

  3112. </div>

  3113. <p>A fix here would be to swap the last two lines (or simply remove the

  3114. <code class="docutils literal notranslate"><span class="pre">p.join()</span></code> line).</p>

  3115. </div></blockquote>

  3116. <p>Explicitly pass resources to child processes</p>

  3117. <blockquote>

  3118. <div><p>On POSIX using the <em>fork</em> start method, a child process can make

  3119. use of a shared resource created in a parent process using a

  3120. global resource.  However, it is better to pass the object as an

  3121. argument to the constructor for the child process.</p>

  3122. <p>Apart from making the code (potentially) compatible with Windows

  3123. and the other start methods this also ensures that as long as the

  3124. child process is still alive the object will not be garbage

  3125. collected in the parent process.  This might be important if some

  3126. resource is freed when the object is garbage collected in the

  3127. parent process.</p>

  3128. <p>So for instance</p>

  3129. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Lock</span>

  3130. 

  3131. <span class="k">def</span> <span class="nf">f</span><span class="p">():</span>

  3132.     <span class="o">...</span> <span class="n">do</span> <span class="n">something</span> <span class="n">using</span> <span class="s2">&quot;lock&quot;</span> <span class="o">...</span>

  3133. 

  3134. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  3135.     <span class="n">lock</span> <span class="o">=</span> <span class="n">Lock</span><span class="p">()</span>

  3136.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>

  3137.         <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">)</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  3138. </pre></div>

  3139. </div>

  3140. <p>should be rewritten as</p>

  3141. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Lock</span>

  3142. 

  3143. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">l</span><span class="p">):</span>

  3144.     <span class="o">...</span> <span class="n">do</span> <span class="n">something</span> <span class="n">using</span> <span class="s2">&quot;l&quot;</span> <span class="o">...</span>

  3145. 

  3146. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  3147.     <span class="n">lock</span> <span class="o">=</span> <span class="n">Lock</span><span class="p">()</span>

  3148.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>

  3149.         <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">f</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">lock</span><span class="p">,))</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  3150. </pre></div>

  3151. </div>

  3152. </div></blockquote>

  3153. <p>Beware of replacing <a class="reference internal" href="sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdin</span></code></a> with a “file like object”</p>

  3154. <blockquote>

  3155. <div><p><a class="reference internal" href="#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> originally unconditionally called:</p>

  3156. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">os</span><span class="o">.</span><span class="n">close</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">stdin</span><span class="o">.</span><span class="n">fileno</span><span class="p">())</span>

  3157. </pre></div>

  3158. </div>

  3159. <p>in the <code class="xref py py-meth docutils literal notranslate"><span class="pre">multiprocessing.Process._bootstrap()</span></code> method — this resulted

  3160. in issues with processes-in-processes. This has been changed to:</p>

  3161. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">sys</span><span class="o">.</span><span class="n">stdin</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>

  3162. <span class="n">sys</span><span class="o">.</span><span class="n">stdin</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">devnull</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">O_RDONLY</span><span class="p">),</span> <span class="n">closefd</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>

  3163. </pre></div>

  3164. </div>

  3165. <p>Which solves the fundamental issue of processes colliding with each other

  3166. resulting in a bad file descriptor error, but introduces a potential danger

  3167. to applications which replace <a class="reference internal" href="sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.stdin()</span></code></a> with a “file-like object”

  3168. with output buffering.  This danger is that if multiple processes call

  3169. <a class="reference internal" href="io.html#io.IOBase.close" title="io.IOBase.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> on this file-like object, it could result in the same

  3170. data being flushed to the object multiple times, resulting in corruption.</p>

  3171. <p>If you write a file-like object and implement your own caching, you can

  3172. make it fork-safe by storing the pid whenever you append to the cache,

  3173. and discarding the cache when the pid changes. For example:</p>

  3174. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@property</span>

  3175. <span class="k">def</span> <span class="nf">cache</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  3176.     <span class="n">pid</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">getpid</span><span class="p">()</span>

  3177.     <span class="k">if</span> <span class="n">pid</span> <span class="o">!=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_pid</span><span class="p">:</span>

  3178.         <span class="bp">self</span><span class="o">.</span><span class="n">_pid</span> <span class="o">=</span> <span class="n">pid</span>

  3179.         <span class="bp">self</span><span class="o">.</span><span class="n">_cache</span> <span class="o">=</span> <span class="p">[]</span>

  3180.     <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache</span>

  3181. </pre></div>

  3182. </div>

  3183. <p>For more information, see <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5155">bpo-5155</a>, <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5313">bpo-5313</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5331">bpo-5331</a></p>

  3184. </div></blockquote>

  3185. </section>

  3186. <section id="the-spawn-and-forkserver-start-methods">

  3187. <h3>The <em>spawn</em> and <em>forkserver</em> start methods<a class="headerlink" href="#the-spawn-and-forkserver-start-methods" title="Link to this heading">¶</a></h3>

  3188. <p>There are a few extra restrictions which don’t apply to the <em>fork</em>

  3189. start method.</p>

  3190. <p>More picklability</p>

  3191. <blockquote>

  3192. <div><p>Ensure that all arguments to <code class="xref py py-meth docutils literal notranslate"><span class="pre">Process.__init__()</span></code> are picklable.

  3193. Also, if you subclass <a class="reference internal" href="#multiprocessing.Process" title="multiprocessing.Process"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code></a> then make sure that

  3194. instances will be picklable when the <a class="reference internal" href="#multiprocessing.Process.start" title="multiprocessing.Process.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Process.start</span></code></a> method is called.</p>

  3195. </div></blockquote>

  3196. <p>Global variables</p>

  3197. <blockquote>

  3198. <div><p>Bear in mind that if code run in a child process tries to access a global

  3199. variable, then the value it sees (if any) may not be the same as the value

  3200. in the parent process at the time that <a class="reference internal" href="#multiprocessing.Process.start" title="multiprocessing.Process.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Process.start</span></code></a> was called.</p>

  3201. <p>However, global variables which are just module level constants cause no

  3202. problems.</p>

  3203. </div></blockquote>

  3204. <p id="multiprocessing-safe-main-import">Safe importing of main module</p>

  3205. <blockquote>

  3206. <div><p>Make sure that the main module can be safely imported by a new Python

  3207. interpreter without causing unintended side effects (such as starting a new

  3208. process).</p>

  3209. <p>For example, using the <em>spawn</em> or <em>forkserver</em> start method

  3210. running the following module would fail with a

  3211. <a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a>:</p>

  3212. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span>

  3213. 

  3214. <span class="k">def</span> <span class="nf">foo</span><span class="p">():</span>

  3215.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;hello&#39;</span><span class="p">)</span>

  3216. 

  3217. <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">foo</span><span class="p">)</span>

  3218. <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  3219. </pre></div>

  3220. </div>

  3221. <p>Instead one should protect the “entry point” of the program by using <code class="docutils literal notranslate"><span class="pre">if</span>

  3222. <span class="pre">__name__</span> <span class="pre">==</span> <span class="pre">'__main__':</span></code> as follows:</p>

  3223. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">freeze_support</span><span class="p">,</span> <span class="n">set_start_method</span>

  3224. 

  3225. <span class="k">def</span> <span class="nf">foo</span><span class="p">():</span>

  3226.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;hello&#39;</span><span class="p">)</span>

  3227. 

  3228. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  3229.     <span class="n">freeze_support</span><span class="p">()</span>

  3230.     <span class="n">set_start_method</span><span class="p">(</span><span class="s1">&#39;spawn&#39;</span><span class="p">)</span>

  3231.     <span class="n">p</span> <span class="o">=</span> <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">foo</span><span class="p">)</span>

  3232.     <span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  3233. </pre></div>

  3234. </div>

  3235. <p>(The <code class="docutils literal notranslate"><span class="pre">freeze_support()</span></code> line can be omitted if the program will be run

  3236. normally instead of frozen.)</p>

  3237. <p>This allows the newly spawned Python interpreter to safely import the module

  3238. and then run the module’s <code class="docutils literal notranslate"><span class="pre">foo()</span></code> function.</p>

  3239. <p>Similar restrictions apply if a pool or manager is created in the main

  3240. module.</p>

  3241. </div></blockquote>

  3242. </section>

  3243. </section>

  3244. <section id="examples">

  3245. <span id="multiprocessing-examples"></span><h2>Examples<a class="headerlink" href="#examples" title="Link to this heading">¶</a></h2>

  3246. <p>Demonstration of how to create and use customized managers and proxies:</p>

  3247. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">freeze_support</span>

  3248. <span class="kn">from</span> <span class="nn">multiprocessing.managers</span> <span class="kn">import</span> <span class="n">BaseManager</span><span class="p">,</span> <span class="n">BaseProxy</span>

  3249. <span class="kn">import</span> <span class="nn">operator</span>

  3250. 

  3251. <span class="c1">##</span>

  3252. 

  3253. <span class="k">class</span> <span class="nc">Foo</span><span class="p">:</span>

  3254.     <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  3255.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;you called Foo.f()&#39;</span><span class="p">)</span>

  3256.     <span class="k">def</span> <span class="nf">g</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  3257.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;you called Foo.g()&#39;</span><span class="p">)</span>

  3258.     <span class="k">def</span> <span class="nf">_h</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  3259.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;you called Foo._h()&#39;</span><span class="p">)</span>

  3260. 

  3261. <span class="c1"># A simple generator function</span>

  3262. <span class="k">def</span> <span class="nf">baz</span><span class="p">():</span>

  3263.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>

  3264.         <span class="k">yield</span> <span class="n">i</span><span class="o">*</span><span class="n">i</span>

  3265. 

  3266. <span class="c1"># Proxy type for generator objects</span>

  3267. <span class="k">class</span> <span class="nc">GeneratorProxy</span><span class="p">(</span><span class="n">BaseProxy</span><span class="p">):</span>

  3268.     <span class="n">_exposed_</span> <span class="o">=</span> <span class="p">[</span><span class="s1">&#39;__next__&#39;</span><span class="p">]</span>

  3269.     <span class="k">def</span> <span class="fm">__iter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  3270.         <span class="k">return</span> <span class="bp">self</span>

  3271.     <span class="k">def</span> <span class="fm">__next__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  3272.         <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_callmethod</span><span class="p">(</span><span class="s1">&#39;__next__&#39;</span><span class="p">)</span>

  3273. 

  3274. <span class="c1"># Function to return the operator module</span>

  3275. <span class="k">def</span> <span class="nf">get_operator_module</span><span class="p">():</span>

  3276.     <span class="k">return</span> <span class="n">operator</span>

  3277. 

  3278. <span class="c1">##</span>

  3279. 

  3280. <span class="k">class</span> <span class="nc">MyManager</span><span class="p">(</span><span class="n">BaseManager</span><span class="p">):</span>

  3281.     <span class="k">pass</span>

  3282. 

  3283. <span class="c1"># register the Foo class; make `f()` and `g()` accessible via proxy</span>

  3284. <span class="n">MyManager</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">&#39;Foo1&#39;</span><span class="p">,</span> <span class="n">Foo</span><span class="p">)</span>

  3285. 

  3286. <span class="c1"># register the Foo class; make `g()` and `_h()` accessible via proxy</span>

  3287. <span class="n">MyManager</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">&#39;Foo2&#39;</span><span class="p">,</span> <span class="n">Foo</span><span class="p">,</span> <span class="n">exposed</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;g&#39;</span><span class="p">,</span> <span class="s1">&#39;_h&#39;</span><span class="p">))</span>

  3288. 

  3289. <span class="c1"># register the generator function baz; use `GeneratorProxy` to make proxies</span>

  3290. <span class="n">MyManager</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">&#39;baz&#39;</span><span class="p">,</span> <span class="n">baz</span><span class="p">,</span> <span class="n">proxytype</span><span class="o">=</span><span class="n">GeneratorProxy</span><span class="p">)</span>

  3291. 

  3292. <span class="c1"># register get_operator_module(); make public functions accessible via proxy</span>

  3293. <span class="n">MyManager</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">&#39;operator&#39;</span><span class="p">,</span> <span class="n">get_operator_module</span><span class="p">)</span>

  3294. 

  3295. <span class="c1">##</span>

  3296. 

  3297. <span class="k">def</span> <span class="nf">test</span><span class="p">():</span>

  3298.     <span class="n">manager</span> <span class="o">=</span> <span class="n">MyManager</span><span class="p">()</span>

  3299.     <span class="n">manager</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  3300. 

  3301.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;-&#39;</span> <span class="o">*</span> <span class="mi">20</span><span class="p">)</span>

  3302. 

  3303.     <span class="n">f1</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">Foo1</span><span class="p">()</span>

  3304.     <span class="n">f1</span><span class="o">.</span><span class="n">f</span><span class="p">()</span>

  3305.     <span class="n">f1</span><span class="o">.</span><span class="n">g</span><span class="p">()</span>

  3306.     <span class="k">assert</span> <span class="ow">not</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">f1</span><span class="p">,</span> <span class="s1">&#39;_h&#39;</span><span class="p">)</span>

  3307.     <span class="k">assert</span> <span class="nb">sorted</span><span class="p">(</span><span class="n">f1</span><span class="o">.</span><span class="n">_exposed_</span><span class="p">)</span> <span class="o">==</span> <span class="nb">sorted</span><span class="p">([</span><span class="s1">&#39;f&#39;</span><span class="p">,</span> <span class="s1">&#39;g&#39;</span><span class="p">])</span>

  3308. 

  3309.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;-&#39;</span> <span class="o">*</span> <span class="mi">20</span><span class="p">)</span>

  3310. 

  3311.     <span class="n">f2</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">Foo2</span><span class="p">()</span>

  3312.     <span class="n">f2</span><span class="o">.</span><span class="n">g</span><span class="p">()</span>

  3313.     <span class="n">f2</span><span class="o">.</span><span class="n">_h</span><span class="p">()</span>

  3314.     <span class="k">assert</span> <span class="ow">not</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">f2</span><span class="p">,</span> <span class="s1">&#39;f&#39;</span><span class="p">)</span>

  3315.     <span class="k">assert</span> <span class="nb">sorted</span><span class="p">(</span><span class="n">f2</span><span class="o">.</span><span class="n">_exposed_</span><span class="p">)</span> <span class="o">==</span> <span class="nb">sorted</span><span class="p">([</span><span class="s1">&#39;g&#39;</span><span class="p">,</span> <span class="s1">&#39;_h&#39;</span><span class="p">])</span>

  3316. 

  3317.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;-&#39;</span> <span class="o">*</span> <span class="mi">20</span><span class="p">)</span>

  3318. 

  3319.     <span class="n">it</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">baz</span><span class="p">()</span>

  3320.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">it</span><span class="p">:</span>

  3321.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;&lt;</span><span class="si">%d</span><span class="s1">&gt;&#39;</span> <span class="o">%</span> <span class="n">i</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">&#39; &#39;</span><span class="p">)</span>

  3322.     <span class="nb">print</span><span class="p">()</span>

  3323. 

  3324.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;-&#39;</span> <span class="o">*</span> <span class="mi">20</span><span class="p">)</span>

  3325. 

  3326.     <span class="n">op</span> <span class="o">=</span> <span class="n">manager</span><span class="o">.</span><span class="n">operator</span><span class="p">()</span>

  3327.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;op.add(23, 45) =&#39;</span><span class="p">,</span> <span class="n">op</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="mi">23</span><span class="p">,</span> <span class="mi">45</span><span class="p">))</span>

  3328.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;op.pow(2, 94) =&#39;</span><span class="p">,</span> <span class="n">op</span><span class="o">.</span><span class="n">pow</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">94</span><span class="p">))</span>

  3329.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;op._exposed_ =&#39;</span><span class="p">,</span> <span class="n">op</span><span class="o">.</span><span class="n">_exposed_</span><span class="p">)</span>

  3330. 

  3331. <span class="c1">##</span>

  3332. 

  3333. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  3334.     <span class="n">freeze_support</span><span class="p">()</span>

  3335.     <span class="n">test</span><span class="p">()</span>

  3336. </pre></div>

  3337. </div>

  3338. <p>Using <a class="reference internal" href="#multiprocessing.pool.Pool" title="multiprocessing.pool.Pool"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pool</span></code></a>:</p>

  3339. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">multiprocessing</span>

  3340. <span class="kn">import</span> <span class="nn">time</span>

  3341. <span class="kn">import</span> <span class="nn">random</span>

  3342. <span class="kn">import</span> <span class="nn">sys</span>

  3343. 

  3344. <span class="c1">#</span>

  3345. <span class="c1"># Functions used by test code</span>

  3346. <span class="c1">#</span>

  3347. 

  3348. <span class="k">def</span> <span class="nf">calculate</span><span class="p">(</span><span class="n">func</span><span class="p">,</span> <span class="n">args</span><span class="p">):</span>

  3349.     <span class="n">result</span> <span class="o">=</span> <span class="n">func</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">)</span>

  3350.     <span class="k">return</span> <span class="s1">&#39;</span><span class="si">%s</span><span class="s1"> says that </span><span class="si">%s%s</span><span class="s1"> = </span><span class="si">%s</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="p">(</span>

  3351.         <span class="n">multiprocessing</span><span class="o">.</span><span class="n">current_process</span><span class="p">()</span><span class="o">.</span><span class="n">name</span><span class="p">,</span>

  3352.         <span class="n">func</span><span class="o">.</span><span class="vm">__name__</span><span class="p">,</span> <span class="n">args</span><span class="p">,</span> <span class="n">result</span>

  3353.         <span class="p">)</span>

  3354. 

  3355. <span class="k">def</span> <span class="nf">calculatestar</span><span class="p">(</span><span class="n">args</span><span class="p">):</span>

  3356.     <span class="k">return</span> <span class="n">calculate</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">)</span>

  3357. 

  3358. <span class="k">def</span> <span class="nf">mul</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span>

  3359.     <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span> <span class="o">*</span> <span class="n">random</span><span class="o">.</span><span class="n">random</span><span class="p">())</span>

  3360.     <span class="k">return</span> <span class="n">a</span> <span class="o">*</span> <span class="n">b</span>

  3361. 

  3362. <span class="k">def</span> <span class="nf">plus</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span>

  3363.     <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span> <span class="o">*</span> <span class="n">random</span><span class="o">.</span><span class="n">random</span><span class="p">())</span>

  3364.     <span class="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>

  3365. 

  3366. <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>

  3367.     <span class="k">return</span> <span class="mf">1.0</span> <span class="o">/</span> <span class="p">(</span><span class="n">x</span> <span class="o">-</span> <span class="mf">5.0</span><span class="p">)</span>

  3368. 

  3369. <span class="k">def</span> <span class="nf">pow3</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>

  3370.     <span class="k">return</span> <span class="n">x</span> <span class="o">**</span> <span class="mi">3</span>

  3371. 

  3372. <span class="k">def</span> <span class="nf">noop</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>

  3373.     <span class="k">pass</span>

  3374. 

  3375. <span class="c1">#</span>

  3376. <span class="c1"># Test code</span>

  3377. <span class="c1">#</span>

  3378. 

  3379. <span class="k">def</span> <span class="nf">test</span><span class="p">():</span>

  3380.     <span class="n">PROCESSES</span> <span class="o">=</span> <span class="mi">4</span>

  3381.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Creating pool with </span><span class="si">%d</span><span class="s1"> processes</span><span class="se">\n</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">PROCESSES</span><span class="p">)</span>

  3382. 

  3383.     <span class="k">with</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">Pool</span><span class="p">(</span><span class="n">PROCESSES</span><span class="p">)</span> <span class="k">as</span> <span class="n">pool</span><span class="p">:</span>

  3384.         <span class="c1">#</span>

  3385.         <span class="c1"># Tests</span>

  3386.         <span class="c1">#</span>

  3387. 

  3388.         <span class="n">TASKS</span> <span class="o">=</span> <span class="p">[(</span><span class="n">mul</span><span class="p">,</span> <span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="mi">7</span><span class="p">))</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)]</span> <span class="o">+</span> \

  3389.                 <span class="p">[(</span><span class="n">plus</span><span class="p">,</span> <span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="mi">8</span><span class="p">))</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)]</span>

  3390. 

  3391.         <span class="n">results</span> <span class="o">=</span> <span class="p">[</span><span class="n">pool</span><span class="o">.</span><span class="n">apply_async</span><span class="p">(</span><span class="n">calculate</span><span class="p">,</span> <span class="n">t</span><span class="p">)</span> <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">TASKS</span><span class="p">]</span>

  3392.         <span class="n">imap_it</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">imap</span><span class="p">(</span><span class="n">calculatestar</span><span class="p">,</span> <span class="n">TASKS</span><span class="p">)</span>

  3393.         <span class="n">imap_unordered_it</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">imap_unordered</span><span class="p">(</span><span class="n">calculatestar</span><span class="p">,</span> <span class="n">TASKS</span><span class="p">)</span>

  3394. 

  3395.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Ordered results using pool.apply_async():&#39;</span><span class="p">)</span>

  3396.         <span class="k">for</span> <span class="n">r</span> <span class="ow">in</span> <span class="n">results</span><span class="p">:</span>

  3397.             <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">r</span><span class="o">.</span><span class="n">get</span><span class="p">())</span>

  3398.         <span class="nb">print</span><span class="p">()</span>

  3399. 

  3400.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Ordered results using pool.imap():&#39;</span><span class="p">)</span>

  3401.         <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">imap_it</span><span class="p">:</span>

  3402.             <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">x</span><span class="p">)</span>

  3403.         <span class="nb">print</span><span class="p">()</span>

  3404. 

  3405.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Unordered results using pool.imap_unordered():&#39;</span><span class="p">)</span>

  3406.         <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">imap_unordered_it</span><span class="p">:</span>

  3407.             <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">x</span><span class="p">)</span>

  3408.         <span class="nb">print</span><span class="p">()</span>

  3409. 

  3410.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Ordered results using pool.map() --- will block till complete:&#39;</span><span class="p">)</span>

  3411.         <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">pool</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="n">calculatestar</span><span class="p">,</span> <span class="n">TASKS</span><span class="p">):</span>

  3412.             <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">x</span><span class="p">)</span>

  3413.         <span class="nb">print</span><span class="p">()</span>

  3414. 

  3415.         <span class="c1">#</span>

  3416.         <span class="c1"># Test error handling</span>

  3417.         <span class="c1">#</span>

  3418. 

  3419.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Testing error handling:&#39;</span><span class="p">)</span>

  3420. 

  3421.         <span class="k">try</span><span class="p">:</span>

  3422.             <span class="nb">print</span><span class="p">(</span><span class="n">pool</span><span class="o">.</span><span class="n">apply</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="p">(</span><span class="mi">5</span><span class="p">,)))</span>

  3423.         <span class="k">except</span> <span class="ne">ZeroDivisionError</span><span class="p">:</span>

  3424.             <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">Got ZeroDivisionError as expected from pool.apply()&#39;</span><span class="p">)</span>

  3425.         <span class="k">else</span><span class="p">:</span>

  3426.             <span class="k">raise</span> <span class="ne">AssertionError</span><span class="p">(</span><span class="s1">&#39;expected ZeroDivisionError&#39;</span><span class="p">)</span>

  3427. 

  3428.         <span class="k">try</span><span class="p">:</span>

  3429.             <span class="nb">print</span><span class="p">(</span><span class="n">pool</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))))</span>

  3430.         <span class="k">except</span> <span class="ne">ZeroDivisionError</span><span class="p">:</span>

  3431.             <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">Got ZeroDivisionError as expected from pool.map()&#39;</span><span class="p">)</span>

  3432.         <span class="k">else</span><span class="p">:</span>

  3433.             <span class="k">raise</span> <span class="ne">AssertionError</span><span class="p">(</span><span class="s1">&#39;expected ZeroDivisionError&#39;</span><span class="p">)</span>

  3434. 

  3435.         <span class="k">try</span><span class="p">:</span>

  3436.             <span class="nb">print</span><span class="p">(</span><span class="nb">list</span><span class="p">(</span><span class="n">pool</span><span class="o">.</span><span class="n">imap</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)))))</span>

  3437.         <span class="k">except</span> <span class="ne">ZeroDivisionError</span><span class="p">:</span>

  3438.             <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">Got ZeroDivisionError as expected from list(pool.imap())&#39;</span><span class="p">)</span>

  3439.         <span class="k">else</span><span class="p">:</span>

  3440.             <span class="k">raise</span> <span class="ne">AssertionError</span><span class="p">(</span><span class="s1">&#39;expected ZeroDivisionError&#39;</span><span class="p">)</span>

  3441. 

  3442.         <span class="n">it</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">imap</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)))</span>

  3443.         <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>

  3444.             <span class="k">try</span><span class="p">:</span>

  3445.                 <span class="n">x</span> <span class="o">=</span> <span class="nb">next</span><span class="p">(</span><span class="n">it</span><span class="p">)</span>

  3446.             <span class="k">except</span> <span class="ne">ZeroDivisionError</span><span class="p">:</span>

  3447.                 <span class="k">if</span> <span class="n">i</span> <span class="o">==</span> <span class="mi">5</span><span class="p">:</span>

  3448.                     <span class="k">pass</span>

  3449.             <span class="k">except</span> <span class="ne">StopIteration</span><span class="p">:</span>

  3450.                 <span class="k">break</span>

  3451.             <span class="k">else</span><span class="p">:</span>

  3452.                 <span class="k">if</span> <span class="n">i</span> <span class="o">==</span> <span class="mi">5</span><span class="p">:</span>

  3453.                     <span class="k">raise</span> <span class="ne">AssertionError</span><span class="p">(</span><span class="s1">&#39;expected ZeroDivisionError&#39;</span><span class="p">)</span>

  3454. 

  3455.         <span class="k">assert</span> <span class="n">i</span> <span class="o">==</span> <span class="mi">9</span>

  3456.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">Got ZeroDivisionError as expected from IMapIterator.next()&#39;</span><span class="p">)</span>

  3457.         <span class="nb">print</span><span class="p">()</span>

  3458. 

  3459.         <span class="c1">#</span>

  3460.         <span class="c1"># Testing timeouts</span>

  3461.         <span class="c1">#</span>

  3462. 

  3463.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Testing ApplyResult.get() with timeout:&#39;</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">&#39; &#39;</span><span class="p">)</span>

  3464.         <span class="n">res</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">apply_async</span><span class="p">(</span><span class="n">calculate</span><span class="p">,</span> <span class="n">TASKS</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>

  3465.         <span class="k">while</span> <span class="mi">1</span><span class="p">:</span>

  3466.             <span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">flush</span><span class="p">()</span>

  3467.             <span class="k">try</span><span class="p">:</span>

  3468.                 <span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\n\t</span><span class="si">%s</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">res</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="mf">0.02</span><span class="p">))</span>

  3469.                 <span class="k">break</span>

  3470.             <span class="k">except</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">TimeoutError</span><span class="p">:</span>

  3471.                 <span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">&#39;.&#39;</span><span class="p">)</span>

  3472.         <span class="nb">print</span><span class="p">()</span>

  3473.         <span class="nb">print</span><span class="p">()</span>

  3474. 

  3475.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Testing IMapIterator.next() with timeout:&#39;</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">&#39; &#39;</span><span class="p">)</span>

  3476.         <span class="n">it</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">imap</span><span class="p">(</span><span class="n">calculatestar</span><span class="p">,</span> <span class="n">TASKS</span><span class="p">)</span>

  3477.         <span class="k">while</span> <span class="mi">1</span><span class="p">:</span>

  3478.             <span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">flush</span><span class="p">()</span>

  3479.             <span class="k">try</span><span class="p">:</span>

  3480.                 <span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\n\t</span><span class="si">%s</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">it</span><span class="o">.</span><span class="n">next</span><span class="p">(</span><span class="mf">0.02</span><span class="p">))</span>

  3481.             <span class="k">except</span> <span class="ne">StopIteration</span><span class="p">:</span>

  3482.                 <span class="k">break</span>

  3483.             <span class="k">except</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">TimeoutError</span><span class="p">:</span>

  3484.                 <span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">&#39;.&#39;</span><span class="p">)</span>

  3485.         <span class="nb">print</span><span class="p">()</span>

  3486.         <span class="nb">print</span><span class="p">()</span>

  3487. 

  3488. 

  3489. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  3490.     <span class="n">multiprocessing</span><span class="o">.</span><span class="n">freeze_support</span><span class="p">()</span>

  3491.     <span class="n">test</span><span class="p">()</span>

  3492. </pre></div>

  3493. </div>

  3494. <p>An example showing how to use queues to feed tasks to a collection of worker

  3495. processes and collect the results:</p>

  3496. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">time</span>

  3497. <span class="kn">import</span> <span class="nn">random</span>

  3498. 

  3499. <span class="kn">from</span> <span class="nn">multiprocessing</span> <span class="kn">import</span> <span class="n">Process</span><span class="p">,</span> <span class="n">Queue</span><span class="p">,</span> <span class="n">current_process</span><span class="p">,</span> <span class="n">freeze_support</span>

  3500. 

  3501. <span class="c1">#</span>

  3502. <span class="c1"># Function run by worker processes</span>

  3503. <span class="c1">#</span>

  3504. 

  3505. <span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">output</span><span class="p">):</span>

  3506.     <span class="k">for</span> <span class="n">func</span><span class="p">,</span> <span class="n">args</span> <span class="ow">in</span> <span class="nb">iter</span><span class="p">(</span><span class="nb">input</span><span class="o">.</span><span class="n">get</span><span class="p">,</span> <span class="s1">&#39;STOP&#39;</span><span class="p">):</span>

  3507.         <span class="n">result</span> <span class="o">=</span> <span class="n">calculate</span><span class="p">(</span><span class="n">func</span><span class="p">,</span> <span class="n">args</span><span class="p">)</span>

  3508.         <span class="n">output</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">result</span><span class="p">)</span>

  3509. 

  3510. <span class="c1">#</span>

  3511. <span class="c1"># Function used to calculate result</span>

  3512. <span class="c1">#</span>

  3513. 

  3514. <span class="k">def</span> <span class="nf">calculate</span><span class="p">(</span><span class="n">func</span><span class="p">,</span> <span class="n">args</span><span class="p">):</span>

  3515.     <span class="n">result</span> <span class="o">=</span> <span class="n">func</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">)</span>

  3516.     <span class="k">return</span> <span class="s1">&#39;</span><span class="si">%s</span><span class="s1"> says that </span><span class="si">%s%s</span><span class="s1"> = </span><span class="si">%s</span><span class="s1">&#39;</span> <span class="o">%</span> \

  3517.         <span class="p">(</span><span class="n">current_process</span><span class="p">()</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">func</span><span class="o">.</span><span class="vm">__name__</span><span class="p">,</span> <span class="n">args</span><span class="p">,</span> <span class="n">result</span><span class="p">)</span>

  3518. 

  3519. <span class="c1">#</span>

  3520. <span class="c1"># Functions referenced by tasks</span>

  3521. <span class="c1">#</span>

  3522. 

  3523. <span class="k">def</span> <span class="nf">mul</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span>

  3524.     <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span><span class="o">*</span><span class="n">random</span><span class="o">.</span><span class="n">random</span><span class="p">())</span>

  3525.     <span class="k">return</span> <span class="n">a</span> <span class="o">*</span> <span class="n">b</span>

  3526. 

  3527. <span class="k">def</span> <span class="nf">plus</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span>

  3528.     <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span><span class="o">*</span><span class="n">random</span><span class="o">.</span><span class="n">random</span><span class="p">())</span>

  3529.     <span class="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>

  3530. 

  3531. <span class="c1">#</span>

  3532. <span class="c1">#</span>

  3533. <span class="c1">#</span>

  3534. 

  3535. <span class="k">def</span> <span class="nf">test</span><span class="p">():</span>

  3536.     <span class="n">NUMBER_OF_PROCESSES</span> <span class="o">=</span> <span class="mi">4</span>

  3537.     <span class="n">TASKS1</span> <span class="o">=</span> <span class="p">[(</span><span class="n">mul</span><span class="p">,</span> <span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="mi">7</span><span class="p">))</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">20</span><span class="p">)]</span>

  3538.     <span class="n">TASKS2</span> <span class="o">=</span> <span class="p">[(</span><span class="n">plus</span><span class="p">,</span> <span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="mi">8</span><span class="p">))</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)]</span>

  3539. 

  3540.     <span class="c1"># Create queues</span>

  3541.     <span class="n">task_queue</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span>

  3542.     <span class="n">done_queue</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span>

  3543. 

  3544.     <span class="c1"># Submit tasks</span>

  3545.     <span class="k">for</span> <span class="n">task</span> <span class="ow">in</span> <span class="n">TASKS1</span><span class="p">:</span>

  3546.         <span class="n">task_queue</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">task</span><span class="p">)</span>

  3547. 

  3548.     <span class="c1"># Start worker processes</span>

  3549.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">NUMBER_OF_PROCESSES</span><span class="p">):</span>

  3550.         <span class="n">Process</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">task_queue</span><span class="p">,</span> <span class="n">done_queue</span><span class="p">))</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>

  3551. 

  3552.     <span class="c1"># Get and print results</span>

  3553.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Unordered results:&#39;</span><span class="p">)</span>

  3554.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">TASKS1</span><span class="p">)):</span>

  3555.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">done_queue</span><span class="o">.</span><span class="n">get</span><span class="p">())</span>

  3556. 

  3557.     <span class="c1"># Add more tasks using `put()`</span>

  3558.     <span class="k">for</span> <span class="n">task</span> <span class="ow">in</span> <span class="n">TASKS2</span><span class="p">:</span>

  3559.         <span class="n">task_queue</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">task</span><span class="p">)</span>

  3560. 

  3561.     <span class="c1"># Get and print some more results</span>

  3562.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">TASKS2</span><span class="p">)):</span>

  3563.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;</span><span class="se">\t</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">done_queue</span><span class="o">.</span><span class="n">get</span><span class="p">())</span>

  3564. 

  3565.     <span class="c1"># Tell child processes to stop</span>

  3566.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">NUMBER_OF_PROCESSES</span><span class="p">):</span>

  3567.         <span class="n">task_queue</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="s1">&#39;STOP&#39;</span><span class="p">)</span>

  3568. 

  3569. 

  3570. <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>

  3571.     <span class="n">freeze_support</span><span class="p">()</span>

  3572.     <span class="n">test</span><span class="p">()</span>

  3573. </pre></div>

  3574. </div>

  3575. </section>

  3576. </section>

  3577. 

  3578. 

  3579.             <div class="clearer"></div>

  3580.           </div>

  3581.         </div>

  3582.       </div>

  3583.       <div class="sphinxsidebar" role="navigation" aria-label="main navigation">

  3584.         <div class="sphinxsidebarwrapper">

  3585.   <div>

  3586.     <h3><a href="../contents.html">Table of Contents</a></h3>

  3587.     <ul>

  3588. <li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code> — Process-based parallelism</a><ul>

  3589. <li><a class="reference internal" href="#introduction">Introduction</a><ul>

  3590. <li><a class="reference internal" href="#the-process-class">The <code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code> class</a></li>

  3591. <li><a class="reference internal" href="#contexts-and-start-methods">Contexts and start methods</a></li>

  3592. <li><a class="reference internal" href="#exchanging-objects-between-processes">Exchanging objects between processes</a></li>

  3593. <li><a class="reference internal" href="#synchronization-between-processes">Synchronization between processes</a></li>

  3594. <li><a class="reference internal" href="#sharing-state-between-processes">Sharing state between processes</a></li>

  3595. <li><a class="reference internal" href="#using-a-pool-of-workers">Using a pool of workers</a></li>

  3596. </ul>

  3597. </li>

  3598. <li><a class="reference internal" href="#reference">Reference</a><ul>

  3599. <li><a class="reference internal" href="#process-and-exceptions"><code class="xref py py-class docutils literal notranslate"><span class="pre">Process</span></code> and exceptions</a></li>

  3600. <li><a class="reference internal" href="#pipes-and-queues">Pipes and Queues</a></li>

  3601. <li><a class="reference internal" href="#miscellaneous">Miscellaneous</a></li>

  3602. <li><a class="reference internal" href="#connection-objects">Connection Objects</a></li>

  3603. <li><a class="reference internal" href="#synchronization-primitives">Synchronization primitives</a></li>

  3604. <li><a class="reference internal" href="#shared-ctypes-objects">Shared <code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code> Objects</a><ul>

  3605. <li><a class="reference internal" href="#module-multiprocessing.sharedctypes">The <code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.sharedctypes</span></code> module</a></li>

  3606. </ul>

  3607. </li>

  3608. <li><a class="reference internal" href="#managers">Managers</a><ul>

  3609. <li><a class="reference internal" href="#customized-managers">Customized managers</a></li>

  3610. <li><a class="reference internal" href="#using-a-remote-manager">Using a remote manager</a></li>

  3611. </ul>

  3612. </li>

  3613. <li><a class="reference internal" href="#proxy-objects">Proxy Objects</a><ul>

  3614. <li><a class="reference internal" href="#cleanup">Cleanup</a></li>

  3615. </ul>

  3616. </li>

  3617. <li><a class="reference internal" href="#module-multiprocessing.pool">Process Pools</a></li>

  3618. <li><a class="reference internal" href="#module-multiprocessing.connection">Listeners and Clients</a><ul>

  3619. <li><a class="reference internal" href="#address-formats">Address Formats</a></li>

  3620. </ul>

  3621. </li>

  3622. <li><a class="reference internal" href="#authentication-keys">Authentication keys</a></li>

  3623. <li><a class="reference internal" href="#logging">Logging</a></li>

  3624. <li><a class="reference internal" href="#module-multiprocessing.dummy">The <code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.dummy</span></code> module</a></li>

  3625. </ul>

  3626. </li>

  3627. <li><a class="reference internal" href="#programming-guidelines">Programming guidelines</a><ul>

  3628. <li><a class="reference internal" href="#all-start-methods">All start methods</a></li>

  3629. <li><a class="reference internal" href="#the-spawn-and-forkserver-start-methods">The <em>spawn</em> and <em>forkserver</em> start methods</a></li>

  3630. </ul>

  3631. </li>

  3632. <li><a class="reference internal" href="#examples">Examples</a></li>

  3633. </ul>

  3634. </li>

  3635. </ul>

  3636. 

  3637.   </div>

  3638.   <div>

  3639.     <h4>Previous topic</h4>

  3640.     <p class="topless"><a href="threading.html"

  3641.                           title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code> — Thread-based parallelism</a></p>

  3642.   </div>

  3643.   <div>

  3644.     <h4>Next topic</h4>

  3645.     <p class="topless"><a href="multiprocessing.shared_memory.html"

  3646.                           title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing.shared_memory</span></code> — Shared memory for direct access across processes</a></p>

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