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  204.   <section id="module-threading">

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

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

  207. <hr class="docutils" />

  208. <p>This module constructs higher-level threading interfaces on top of the lower

  209. level <a class="reference internal" href="_thread.html#module-_thread" title="_thread: Low-level threading API."><code class="xref py py-mod docutils literal notranslate"><span class="pre">_thread</span></code></a> module.</p>

  210. <div class="versionchanged">

  211. <p><span class="versionmodified changed">Changed in version 3.7: </span>This module used to be optional, it is now always available.</p>

  212. </div>

  213. <div class="admonition seealso">

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

  215. <p><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> offers a higher level interface

  216. to push tasks to a background thread without blocking execution of the

  217. calling thread, while still being able to retrieve their results when needed.</p>

  218. <p><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> provides a thread-safe interface for exchanging data between

  219. running threads.</p>

  220. <p><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> offers an alternative approach to achieving task level

  221. concurrency without requiring the use of multiple operating system threads.</p>

  222. </div>

  223. <div class="admonition note">

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

  225. <p>In the Python 2.x series, this module contained <code class="docutils literal notranslate"><span class="pre">camelCase</span></code> names

  226. for some methods and functions. These are deprecated as of Python 3.10,

  227. but they are still supported for compatibility with Python 2.5 and lower.</p>

  228. </div>

  229. <div class="impl-detail compound">

  230. <p><strong>CPython implementation detail:</strong> In CPython, due to the <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><span class="xref std std-term">Global Interpreter Lock</span></a>, only one thread

  231. can execute Python code at once (even though certain performance-oriented

  232. libraries might overcome this limitation).

  233. If you want your application to make better use of the computational

  234. resources of multi-core machines, you are advised to use

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

  236. However, threading is still an appropriate model if you want to run

  237. multiple I/O-bound tasks simultaneously.</p>

  238. </div>

  239. <div class="availability docutils container">

  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. <p>This module defines the following functions:</p>

  246. <dl class="py function">

  247. <dt class="sig sig-object py" id="threading.active_count">

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

  249. <dd><p>Return the number of <a class="reference internal" href="#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">Thread</span></code></a> objects currently alive.  The returned

  250. count is equal to the length of the list returned by <a class="reference internal" href="#threading.enumerate" title="threading.enumerate"><code class="xref py py-func docutils literal notranslate"><span class="pre">enumerate()</span></code></a>.</p>

  251. <p>The function <code class="docutils literal notranslate"><span class="pre">activeCount</span></code> is a deprecated alias for this function.</p>

  252. </dd></dl>

  253. 

  254. <dl class="py function">

  255. <dt class="sig sig-object py" id="threading.current_thread">

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

  257. <dd><p>Return the current <a class="reference internal" href="#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">Thread</span></code></a> object, corresponding to the caller’s thread

  258. of control.  If the caller’s thread of control was not created through the

  259. <a class="reference internal" href="#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 dummy thread object with limited functionality is

  260. returned.</p>

  261. <p>The function <code class="docutils literal notranslate"><span class="pre">currentThread</span></code> is a deprecated alias for this function.</p>

  262. </dd></dl>

  263. 

  264. <dl class="py function">

  265. <dt class="sig sig-object py" id="threading.excepthook">

  266. <span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">excepthook</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">args</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">/</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#threading.excepthook" title="Link to this definition">¶</a></dt>

  267. <dd><p>Handle uncaught exception raised by <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-func docutils literal notranslate"><span class="pre">Thread.run()</span></code></a>.</p>

  268. <p>The <em>args</em> argument has the following attributes:</p>

  269. <ul class="simple">

  270. <li><p><em>exc_type</em>: Exception type.</p></li>

  271. <li><p><em>exc_value</em>: Exception value, can be <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p></li>

  272. <li><p><em>exc_traceback</em>: Exception traceback, can be <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p></li>

  273. <li><p><em>thread</em>: Thread which raised the exception, can be <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p></li>

  274. </ul>

  275. <p>If <em>exc_type</em> is <a class="reference internal" href="exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a>, the exception is silently ignored.

  276. Otherwise, the exception is printed out on <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>.</p>

  277. <p>If  this function raises an exception, <a class="reference internal" href="sys.html#sys.excepthook" title="sys.excepthook"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.excepthook()</span></code></a> is called to

  278. handle it.</p>

  279. <p><a class="reference internal" href="#threading.excepthook" title="threading.excepthook"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.excepthook()</span></code></a> can be overridden to control how uncaught

  280. exceptions raised by <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-func docutils literal notranslate"><span class="pre">Thread.run()</span></code></a> are handled.</p>

  281. <p>Storing <em>exc_value</em> using a custom hook can create a reference cycle. It

  282. should be cleared explicitly to break the reference cycle when the

  283. exception is no longer needed.</p>

  284. <p>Storing <em>thread</em> using a custom hook can resurrect it if it is set to an

  285. object which is being finalized. Avoid storing <em>thread</em> after the custom

  286. hook completes to avoid resurrecting objects.</p>

  287. <div class="admonition seealso">

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

  289. <p><a class="reference internal" href="sys.html#sys.excepthook" title="sys.excepthook"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.excepthook()</span></code></a> handles uncaught exceptions.</p>

  290. </div>

  291. <div class="versionadded">

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

  293. </div>

  294. </dd></dl>

  295. 

  296. <dl class="py data">

  297. <dt class="sig sig-object py" id="threading.__excepthook__">

  298. <span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">__excepthook__</span></span><a class="headerlink" href="#threading.__excepthook__" title="Link to this definition">¶</a></dt>

  299. <dd><p>Holds the original value of <a class="reference internal" href="#threading.excepthook" title="threading.excepthook"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.excepthook()</span></code></a>. It is saved so that the

  300. original value can be restored in case they happen to get replaced with

  301. broken or alternative objects.</p>

  302. <div class="versionadded">

  303. <p><span class="versionmodified added">New in version 3.10.</span></p>

  304. </div>

  305. </dd></dl>

  306. 

  307. <dl class="py function">

  308. <dt class="sig sig-object py" id="threading.get_ident">

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

  310. <dd><p>Return the ‘thread identifier’ of the current thread.  This is a nonzero

  311. integer.  Its value has no direct meaning; it is intended as a magic cookie

  312. to be used e.g. to index a dictionary of thread-specific data.  Thread

  313. identifiers may be recycled when a thread exits and another thread is

  314. created.</p>

  315. <div class="versionadded">

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

  317. </div>

  318. </dd></dl>

  319. 

  320. <dl class="py function">

  321. <dt class="sig sig-object py" id="threading.get_native_id">

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

  323. <dd><p>Return the native integral Thread ID of the current thread assigned by the kernel.

  324. This is a non-negative integer.

  325. Its value may be used to uniquely identify this particular thread system-wide

  326. (until the thread terminates, after which the value may be recycled by the OS).</p>

  327. <div class="availability docutils container">

  328. <p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows, FreeBSD, Linux, macOS, OpenBSD, NetBSD, AIX, DragonFlyBSD.</p>

  329. </div>

  330. <div class="versionadded">

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

  332. </div>

  333. </dd></dl>

  334. 

  335. <dl class="py function">

  336. <dt class="sig sig-object py" id="threading.enumerate">

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

  338. <dd><p>Return a list of all <a class="reference internal" href="#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">Thread</span></code></a> objects currently active.  The list

  339. includes daemonic threads and dummy thread objects created by

  340. <a class="reference internal" href="#threading.current_thread" title="threading.current_thread"><code class="xref py py-func docutils literal notranslate"><span class="pre">current_thread()</span></code></a>.  It excludes terminated threads and threads

  341. that have not yet been started.  However, the main thread is always part

  342. of the result, even when terminated.</p>

  343. </dd></dl>

  344. 

  345. <dl class="py function">

  346. <dt class="sig sig-object py" id="threading.main_thread">

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

  348. <dd><p>Return the main <a class="reference internal" href="#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">Thread</span></code></a> object.  In normal conditions, the

  349. main thread is the thread from which the Python interpreter was

  350. started.</p>

  351. <div class="versionadded">

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

  353. </div>

  354. </dd></dl>

  355. 

  356. <dl class="py function">

  357. <dt class="sig sig-object py" id="threading.settrace">

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

  359. <dd><p id="index-0">Set a trace function for all threads started from the <a class="reference internal" href="#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module.

  360. The <em>func</em> will be passed to  <a class="reference internal" href="sys.html#sys.settrace" title="sys.settrace"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.settrace()</span></code></a> for each thread, before its

  361. <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method is called.</p>

  362. </dd></dl>

  363. 

  364. <dl class="py function">

  365. <dt class="sig sig-object py" id="threading.settrace_all_threads">

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

  367. <dd><p>Set a trace function for all threads started from the <a class="reference internal" href="#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module

  368. and all Python threads that are currently executing.</p>

  369. <p>The <em>func</em> will be passed to  <a class="reference internal" href="sys.html#sys.settrace" title="sys.settrace"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.settrace()</span></code></a> for each thread, before its

  370. <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method is called.</p>

  371. <div class="versionadded">

  372. <p><span class="versionmodified added">New in version 3.12.</span></p>

  373. </div>

  374. </dd></dl>

  375. 

  376. <dl class="py function">

  377. <dt class="sig sig-object py" id="threading.gettrace">

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

  379. <dd><p id="index-1">Get the trace function as set by <a class="reference internal" href="#threading.settrace" title="threading.settrace"><code class="xref py py-func docutils literal notranslate"><span class="pre">settrace()</span></code></a>.</p>

  380. <div class="versionadded">

  381. <p><span class="versionmodified added">New in version 3.10.</span></p>

  382. </div>

  383. </dd></dl>

  384. 

  385. <dl class="py function">

  386. <dt class="sig sig-object py" id="threading.setprofile">

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

  388. <dd><p id="index-2">Set a profile function for all threads started from the <a class="reference internal" href="#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module.

  389. The <em>func</em> will be passed to  <a class="reference internal" href="sys.html#sys.setprofile" title="sys.setprofile"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.setprofile()</span></code></a> for each thread, before its

  390. <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method is called.</p>

  391. </dd></dl>

  392. 

  393. <dl class="py function">

  394. <dt class="sig sig-object py" id="threading.setprofile_all_threads">

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

  396. <dd><p>Set a profile function for all threads started from the <a class="reference internal" href="#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module

  397. and all Python threads that are currently executing.</p>

  398. <p>The <em>func</em> will be passed to  <a class="reference internal" href="sys.html#sys.setprofile" title="sys.setprofile"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.setprofile()</span></code></a> for each thread, before its

  399. <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method is called.</p>

  400. <div class="versionadded">

  401. <p><span class="versionmodified added">New in version 3.12.</span></p>

  402. </div>

  403. </dd></dl>

  404. 

  405. <dl class="py function">

  406. <dt class="sig sig-object py" id="threading.getprofile">

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

  408. <dd><p id="index-3">Get the profiler function as set by <a class="reference internal" href="#threading.setprofile" title="threading.setprofile"><code class="xref py py-func docutils literal notranslate"><span class="pre">setprofile()</span></code></a>.</p>

  409. <div class="versionadded">

  410. <p><span class="versionmodified added">New in version 3.10.</span></p>

  411. </div>

  412. </dd></dl>

  413. 

  414. <dl class="py function">

  415. <dt class="sig sig-object py" id="threading.stack_size">

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

  417. <dd><p>Return the thread stack size used when creating new threads.  The optional

  418. <em>size</em> argument specifies the stack size to be used for subsequently created

  419. threads, and must be 0 (use platform or configured default) or a positive

  420. integer value of at least 32,768 (32 KiB). If <em>size</em> is not specified,

  421. 0 is used.  If changing the thread stack size is

  422. unsupported, a <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> is raised.  If the specified stack size is

  423. invalid, 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 and the stack size is unmodified.  32 KiB

  424. is currently the minimum supported stack size value to guarantee sufficient

  425. stack space for the interpreter itself.  Note that some platforms may have

  426. particular restrictions on values for the stack size, such as requiring a

  427. minimum stack size &gt; 32 KiB or requiring allocation in multiples of the system

  428. memory page size - platform documentation should be referred to for more

  429. information (4 KiB pages are common; using multiples of 4096 for the stack size is

  430. the suggested approach in the absence of more specific information).</p>

  431. <div class="availability docutils container">

  432. <p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows, pthreads.</p>

  433. <p>Unix platforms with POSIX threads support.</p>

  434. </div>

  435. </dd></dl>

  436. 

  437. <p>This module also defines the following constant:</p>

  438. <dl class="py data">

  439. <dt class="sig sig-object py" id="threading.TIMEOUT_MAX">

  440. <span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">TIMEOUT_MAX</span></span><a class="headerlink" href="#threading.TIMEOUT_MAX" title="Link to this definition">¶</a></dt>

  441. <dd><p>The maximum value allowed for the <em>timeout</em> parameter of blocking functions

  442. (<a class="reference internal" href="#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Lock.acquire()</span></code></a>, <a class="reference internal" href="#threading.RLock.acquire" title="threading.RLock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">RLock.acquire()</span></code></a>, <a class="reference internal" href="#threading.Condition.wait" title="threading.Condition.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Condition.wait()</span></code></a>, etc.).

  443. Specifying a timeout greater than this value will raise an

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

  445. <div class="versionadded">

  446. <p><span class="versionmodified added">New in version 3.2.</span></p>

  447. </div>

  448. </dd></dl>

  449. 

  450. <p>This module defines a number of classes, which are detailed in the sections

  451. below.</p>

  452. <p>The design of this module is loosely based on Java’s threading model. However,

  453. where Java makes locks and condition variables basic behavior of every object,

  454. they are separate objects in Python.  Python’s <a class="reference internal" href="#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">Thread</span></code></a> class supports a

  455. subset of the behavior of Java’s Thread class; currently, there are no

  456. priorities, no thread groups, and threads cannot be destroyed, stopped,

  457. suspended, resumed, or interrupted.  The static methods of Java’s Thread class,

  458. when implemented, are mapped to module-level functions.</p>

  459. <p>All of the methods described below are executed atomically.</p>

  460. <section id="thread-local-data">

  461. <h2>Thread-Local Data<a class="headerlink" href="#thread-local-data" title="Link to this heading">¶</a></h2>

  462. <p>Thread-local data is data whose values are thread specific.  To manage

  463. thread-local data, just create an instance of <a class="reference internal" href="#threading.local" title="threading.local"><code class="xref py py-class docutils literal notranslate"><span class="pre">local</span></code></a> (or a

  464. subclass) and store attributes on it:</p>

  465. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">mydata</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">local</span><span class="p">()</span>

  466. <span class="n">mydata</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>

  467. </pre></div>

  468. </div>

  469. <p>The instance’s values will be different for separate threads.</p>

  470. <dl class="py class">

  471. <dt class="sig sig-object py" id="threading.local">

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

  473. <dd><p>A class that represents thread-local data.</p>

  474. <p>For more details and extensive examples, see the documentation string of the

  475. <code class="xref py py-mod docutils literal notranslate"><span class="pre">_threading_local</span></code> module: <a class="reference external" href="https://github.com/python/cpython/tree/3.12/Lib/_threading_local.py">Lib/_threading_local.py</a>.</p>

  476. </dd></dl>

  477. 

  478. </section>

  479. <section id="thread-objects">

  480. <span id="id1"></span><h2>Thread Objects<a class="headerlink" href="#thread-objects" title="Link to this heading">¶</a></h2>

  481. <p>The <a class="reference internal" href="#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">Thread</span></code></a> class represents an activity that is run in a separate

  482. thread of control.  There are two ways to specify the activity: by passing a

  483. callable object to the constructor, or by overriding the <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a>

  484. method in a subclass.  No other methods (except for the constructor) should be

  485. overridden in a subclass.  In other words, <em>only</em>  override the

  486. <code class="docutils literal notranslate"><span class="pre">__init__()</span></code> and <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> methods of this class.</p>

  487. <p>Once a thread object is created, its activity must be started by calling the

  488. thread’s <a class="reference internal" href="#threading.Thread.start" title="threading.Thread.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">start()</span></code></a> method.  This invokes the <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a>

  489. method in a separate thread of control.</p>

  490. <p>Once the thread’s activity is started, the thread is considered ‘alive’. It

  491. stops being alive when its <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method terminates – either

  492. normally, or by raising an unhandled exception.  The <a class="reference internal" href="#threading.Thread.is_alive" title="threading.Thread.is_alive"><code class="xref py py-meth docutils literal notranslate"><span class="pre">is_alive()</span></code></a>

  493. method tests whether the thread is alive.</p>

  494. <p>Other threads can call a thread’s <a class="reference internal" href="#threading.Thread.join" title="threading.Thread.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> method.  This blocks

  495. the calling thread until the thread whose <a class="reference internal" href="#threading.Thread.join" title="threading.Thread.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> method is

  496. called is terminated.</p>

  497. <p>A thread has a name.  The name can be passed to the constructor, and read or

  498. changed through the <a class="reference internal" href="#threading.Thread.name" title="threading.Thread.name"><code class="xref py py-attr docutils literal notranslate"><span class="pre">name</span></code></a> attribute.</p>

  499. <p>If the <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method raises an exception,

  500. <a class="reference internal" href="#threading.excepthook" title="threading.excepthook"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.excepthook()</span></code></a> is called to handle it. By default,

  501. <a class="reference internal" href="#threading.excepthook" title="threading.excepthook"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.excepthook()</span></code></a> ignores silently <a class="reference internal" href="exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a>.</p>

  502. <p>A thread can be flagged as a “daemon thread”.  The significance of this flag is

  503. that the entire Python program exits when only daemon threads are left.  The

  504. initial value is inherited from the creating thread.  The flag can be set

  505. through the <a class="reference internal" href="#threading.Thread.daemon" title="threading.Thread.daemon"><code class="xref py py-attr docutils literal notranslate"><span class="pre">daemon</span></code></a> property or the <em>daemon</em> constructor

  506. argument.</p>

  507. <div class="admonition note">

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

  509. <p>Daemon threads are abruptly stopped at shutdown.  Their resources (such

  510. as open files, database transactions, etc.) may not be released properly.

  511. If you want your threads to stop gracefully, make them non-daemonic and

  512. use a suitable signalling mechanism such as an <a class="reference internal" href="#threading.Event" title="threading.Event"><code class="xref py py-class docutils literal notranslate"><span class="pre">Event</span></code></a>.</p>

  513. </div>

  514. <p>There is a “main thread” object; this corresponds to the initial thread of

  515. control in the Python program.  It is not a daemon thread.</p>

  516. <p>There is the possibility that “dummy thread objects” are created. These are

  517. thread objects corresponding to “alien threads”, which are threads of control

  518. started outside the threading module, such as directly from C code.  Dummy

  519. thread objects have limited functionality; they are always considered alive and

  520. daemonic, and cannot be <a class="reference internal" href="#meth-thread-join"><span class="std std-ref">joined</span></a>.  They are never deleted,

  521. since it is impossible to detect the termination of alien threads.</p>

  522. <dl class="py class">

  523. <dt class="sig sig-object py" id="threading.Thread">

  524. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">Thread</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="#threading.Thread" title="Link to this definition">¶</a></dt>

  525. <dd><p>This constructor should always be called with keyword arguments.  Arguments

  526. are:</p>

  527. <p><em>group</em> should be <code class="docutils literal notranslate"><span class="pre">None</span></code>; reserved for future extension when a

  528. <code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadGroup</span></code> class is implemented.</p>

  529. <p><em>target</em> is the callable object to be invoked by the <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method.

  530. Defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>, meaning nothing is called.</p>

  531. <p><em>name</em> is the thread name. By default, a unique name is constructed

  532. of the form “Thread-<em>N</em>” where <em>N</em> is a small decimal number,

  533. or “Thread-<em>N</em> (target)” where “target” is <code class="docutils literal notranslate"><span class="pre">target.__name__</span></code> if the

  534. <em>target</em> argument is specified.</p>

  535. <p><em>args</em> is a list or tuple of arguments for the target invocation.  Defaults to <code class="docutils literal notranslate"><span class="pre">()</span></code>.</p>

  536. <p><em>kwargs</em> is a dictionary of keyword arguments for the target invocation.

  537. Defaults to <code class="docutils literal notranslate"><span class="pre">{}</span></code>.</p>

  538. <p>If not <code class="docutils literal notranslate"><span class="pre">None</span></code>, <em>daemon</em> explicitly sets whether the thread is daemonic.

  539. If <code class="docutils literal notranslate"><span class="pre">None</span></code> (the default), the daemonic property is inherited from the

  540. current thread.</p>

  541. <p>If the subclass overrides the constructor, it must make sure to invoke the

  542. base class constructor (<code class="docutils literal notranslate"><span class="pre">Thread.__init__()</span></code>) before doing anything else to

  543. the thread.</p>

  544. <div class="versionchanged">

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

  546. </div>

  547. <div class="versionchanged">

  548. <p><span class="versionmodified changed">Changed in version 3.10: </span>Use the <em>target</em> name if <em>name</em> argument is omitted.</p>

  549. </div>

  550. <dl class="py method">

  551. <dt class="sig sig-object py" id="threading.Thread.start">

  552. <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="#threading.Thread.start" title="Link to this definition">¶</a></dt>

  553. <dd><p>Start the thread’s activity.</p>

  554. <p>It must be called at most once per thread object.  It arranges for the

  555. object’s <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method to be invoked in a separate thread

  556. of control.</p>

  557. <p>This method will raise a <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 called more than once

  558. on the same thread object.</p>

  559. </dd></dl>

  560. 

  561. <dl class="py method">

  562. <dt class="sig sig-object py" id="threading.Thread.run">

  563. <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="#threading.Thread.run" title="Link to this definition">¶</a></dt>

  564. <dd><p>Method representing the thread’s activity.</p>

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

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

  567. the <em>target</em> argument, if any, with positional and keyword arguments taken

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

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

  570. could achieve the same effect.</p>

  571. <p>Example:</p>

  572. <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">threading</span> <span class="kn">import</span> <span class="n">Thread</span>

  573. <span class="gp">&gt;&gt;&gt; </span><span class="n">t</span> <span class="o">=</span> <span class="n">Thread</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>

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

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

  576. <span class="gp">&gt;&gt;&gt; </span><span class="n">t</span> <span class="o">=</span> <span class="n">Thread</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>

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

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

  579. </pre></div>

  580. </div>

  581. </dd></dl>

  582. 

  583. <dl class="py method" id="meth-thread-join">

  584. <dt class="sig sig-object py" id="threading.Thread.join">

  585. <span class="sig-name descname"><span class="pre">join</span></span><span class="sig-paren">(</span><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="#threading.Thread.join" title="Link to this definition">¶</a></dt>

  586. <dd><p>Wait until the thread terminates. This blocks the calling thread until

  587. the thread whose <a class="reference internal" href="#threading.Thread.join" title="threading.Thread.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> method is called terminates – either

  588. normally or through an unhandled exception – or until the optional

  589. timeout occurs.</p>

  590. <p>When the <em>timeout</em> argument is present and not <code class="docutils literal notranslate"><span class="pre">None</span></code>, it should be a

  591. floating point number specifying a timeout for the operation in seconds

  592. (or fractions thereof). As <a class="reference internal" href="#threading.Thread.join" title="threading.Thread.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> always returns <code class="docutils literal notranslate"><span class="pre">None</span></code>,

  593. you must call <a class="reference internal" href="#threading.Thread.is_alive" title="threading.Thread.is_alive"><code class="xref py py-meth docutils literal notranslate"><span class="pre">is_alive()</span></code></a> after <a class="reference internal" href="#threading.Thread.join" title="threading.Thread.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> to

  594. decide whether a timeout happened – if the thread is still alive, the

  595. <a class="reference internal" href="#threading.Thread.join" title="threading.Thread.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> call timed out.</p>

  596. <p>When the <em>timeout</em> argument is not present or <code class="docutils literal notranslate"><span class="pre">None</span></code>, the operation will

  597. block until the thread terminates.</p>

  598. <p>A thread can be joined many times.</p>

  599. <p><a class="reference internal" href="#threading.Thread.join" title="threading.Thread.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> raises a <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 an attempt is made

  600. to join the current thread as that would cause a deadlock. It is also

  601. an error to <a class="reference internal" href="#threading.Thread.join" title="threading.Thread.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> a thread before it has been started

  602. and attempts to do so raise the same exception.</p>

  603. </dd></dl>

  604. 

  605. <dl class="py attribute">

  606. <dt class="sig sig-object py" id="threading.Thread.name">

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

  608. <dd><p>A string used for identification purposes only. It has no semantics.

  609. Multiple threads may be given the same name.  The initial name is set by

  610. the constructor.</p>

  611. </dd></dl>

  612. 

  613. <dl class="py method">

  614. <dt class="sig sig-object py" id="threading.Thread.getName">

  615. <span class="sig-name descname"><span class="pre">getName</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Thread.getName" title="Link to this definition">¶</a></dt>

  616. <dt class="sig sig-object py" id="threading.Thread.setName">

  617. <span class="sig-name descname"><span class="pre">setName</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Thread.setName" title="Link to this definition">¶</a></dt>

  618. <dd><p>Deprecated getter/setter API for <a class="reference internal" href="#threading.Thread.name" title="threading.Thread.name"><code class="xref py py-attr docutils literal notranslate"><span class="pre">name</span></code></a>; use it directly as a

  619. property instead.</p>

  620. <div class="deprecated">

  621. <p><span class="versionmodified deprecated">Deprecated since version 3.10.</span></p>

  622. </div>

  623. </dd></dl>

  624. 

  625. <dl class="py attribute">

  626. <dt class="sig sig-object py" id="threading.Thread.ident">

  627. <span class="sig-name descname"><span class="pre">ident</span></span><a class="headerlink" href="#threading.Thread.ident" title="Link to this definition">¶</a></dt>

  628. <dd><p>The ‘thread identifier’ of this thread or <code class="docutils literal notranslate"><span class="pre">None</span></code> if the thread has not

  629. been started.  This is a nonzero integer.  See the <a class="reference internal" href="#threading.get_ident" title="threading.get_ident"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_ident()</span></code></a>

  630. function.  Thread identifiers may be recycled when a thread exits and

  631. another thread is created.  The identifier is available even after the

  632. thread has exited.</p>

  633. </dd></dl>

  634. 

  635. <dl class="py attribute">

  636. <dt class="sig sig-object py" id="threading.Thread.native_id">

  637. <span class="sig-name descname"><span class="pre">native_id</span></span><a class="headerlink" href="#threading.Thread.native_id" title="Link to this definition">¶</a></dt>

  638. <dd><p>The Thread ID (<code class="docutils literal notranslate"><span class="pre">TID</span></code>) of this thread, as assigned by the OS (kernel).

  639. This is a non-negative integer, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if the thread has not

  640. been started. See the <a class="reference internal" href="#threading.get_native_id" title="threading.get_native_id"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_native_id()</span></code></a> function.

  641. This value may be used to uniquely identify this particular thread

  642. system-wide (until the thread terminates, after which the value

  643. may be recycled by the OS).</p>

  644. <div class="admonition note">

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

  646. <p>Similar to Process IDs, Thread IDs are only valid (guaranteed unique

  647. system-wide) from the time the thread is created until the thread

  648. has been terminated.</p>

  649. </div>

  650. <div class="availability docutils container">

  651. <p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows, FreeBSD, Linux, macOS, OpenBSD, NetBSD, AIX, DragonFlyBSD.</p>

  652. </div>

  653. <div class="versionadded">

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

  655. </div>

  656. </dd></dl>

  657. 

  658. <dl class="py method">

  659. <dt class="sig sig-object py" id="threading.Thread.is_alive">

  660. <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="#threading.Thread.is_alive" title="Link to this definition">¶</a></dt>

  661. <dd><p>Return whether the thread is alive.</p>

  662. <p>This method returns <code class="docutils literal notranslate"><span class="pre">True</span></code> just before the <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method

  663. starts until just after the <a class="reference internal" href="#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method terminates.  The

  664. module function <a class="reference internal" href="#threading.enumerate" title="threading.enumerate"><code class="xref py py-func docutils literal notranslate"><span class="pre">enumerate()</span></code></a> returns a list of all alive threads.</p>

  665. </dd></dl>

  666. 

  667. <dl class="py attribute">

  668. <dt class="sig sig-object py" id="threading.Thread.daemon">

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

  670. <dd><p>A boolean value indicating whether this thread is a daemon thread (<code class="docutils literal notranslate"><span class="pre">True</span></code>)

  671. or not (<code class="docutils literal notranslate"><span class="pre">False</span></code>).  This must be set before <a class="reference internal" href="#threading.Thread.start" title="threading.Thread.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">start()</span></code></a> is called,

  672. otherwise <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> is raised.  Its initial value is inherited

  673. from the creating thread; the main thread is not a daemon thread and

  674. therefore all threads created in the main thread default to

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

  676. <p>The entire Python program exits when no alive non-daemon threads are left.</p>

  677. </dd></dl>

  678. 

  679. <dl class="py method">

  680. <dt class="sig sig-object py" id="threading.Thread.isDaemon">

  681. <span class="sig-name descname"><span class="pre">isDaemon</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Thread.isDaemon" title="Link to this definition">¶</a></dt>

  682. <dt class="sig sig-object py" id="threading.Thread.setDaemon">

  683. <span class="sig-name descname"><span class="pre">setDaemon</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Thread.setDaemon" title="Link to this definition">¶</a></dt>

  684. <dd><p>Deprecated getter/setter API for <a class="reference internal" href="#threading.Thread.daemon" title="threading.Thread.daemon"><code class="xref py py-attr docutils literal notranslate"><span class="pre">daemon</span></code></a>; use it directly as a

  685. property instead.</p>

  686. <div class="deprecated">

  687. <p><span class="versionmodified deprecated">Deprecated since version 3.10.</span></p>

  688. </div>

  689. </dd></dl>

  690. 

  691. </dd></dl>

  692. 

  693. </section>

  694. <section id="lock-objects">

  695. <span id="id2"></span><h2>Lock Objects<a class="headerlink" href="#lock-objects" title="Link to this heading">¶</a></h2>

  696. <p>A primitive lock is a synchronization primitive that is not owned by a

  697. particular thread when locked.  In Python, it is currently the lowest level

  698. synchronization primitive available, implemented directly by the <a class="reference internal" href="_thread.html#module-_thread" title="_thread: Low-level threading API."><code class="xref py py-mod docutils literal notranslate"><span class="pre">_thread</span></code></a>

  699. extension module.</p>

  700. <p>A primitive lock is in one of two states, “locked” or “unlocked”. It is created

  701. in the unlocked state.  It has two basic methods, <a class="reference internal" href="#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> and

  702. <a class="reference internal" href="#threading.Lock.release" title="threading.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a>.  When the state is unlocked, <a class="reference internal" href="#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a>

  703. changes the state to locked and returns immediately.  When the state is locked,

  704. <a class="reference internal" href="#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> blocks until a call to <a class="reference internal" href="#threading.Lock.release" title="threading.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> in another

  705. thread changes it to unlocked, then the <a class="reference internal" href="#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> call resets it

  706. to locked and returns.  The <a class="reference internal" href="#threading.Lock.release" title="threading.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> method should only be

  707. called in the locked state; it changes the state to unlocked and returns

  708. immediately. If an attempt is made to release an unlocked lock, a

  709. <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> will be raised.</p>

  710. <p>Locks also support the <a class="reference internal" href="#with-locks"><span class="std std-ref">context management protocol</span></a>.</p>

  711. <p>When more than one thread is blocked in <a class="reference internal" href="#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> waiting for the

  712. state to turn to unlocked, only one thread proceeds when a <a class="reference internal" href="#threading.Lock.release" title="threading.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a>

  713. call resets the state to unlocked; which one of the waiting threads proceeds

  714. is not defined, and may vary across implementations.</p>

  715. <p>All methods are executed atomically.</p>

  716. <dl class="py class">

  717. <dt class="sig sig-object py" id="threading.Lock">

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

  719. <dd><p>The class implementing primitive lock objects.  Once a thread has acquired a

  720. lock, subsequent attempts to acquire it block, until it is released; any

  721. thread may release it.</p>

  722. <p>Note that <code class="docutils literal notranslate"><span class="pre">Lock</span></code> is actually a factory function which returns an instance

  723. of the most efficient version of the concrete Lock class that is supported

  724. by the platform.</p>

  725. <dl class="py method">

  726. <dt class="sig sig-object py" id="threading.Lock.acquire">

  727. <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">blocking</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">-1</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#threading.Lock.acquire" title="Link to this definition">¶</a></dt>

  728. <dd><p>Acquire a lock, blocking or non-blocking.</p>

  729. <p>When invoked with the <em>blocking</em> argument set to <code class="docutils literal notranslate"><span class="pre">True</span></code> (the default),

  730. block until the lock is unlocked, then set it to locked and return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  731. <p>When invoked with the <em>blocking</em> argument set to <code class="docutils literal notranslate"><span class="pre">False</span></code>, do not block.

  732. If a call with <em>blocking</em> set to <code class="docutils literal notranslate"><span class="pre">True</span></code> would block, return <code class="docutils literal notranslate"><span class="pre">False</span></code>

  733. immediately; otherwise, set the lock to locked and return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  734. <p>When invoked with the floating-point <em>timeout</em> argument set to a positive

  735. value, block for at most the number of seconds specified by <em>timeout</em>

  736. and as long as the lock cannot be acquired.  A <em>timeout</em> argument of <code class="docutils literal notranslate"><span class="pre">-1</span></code>

  737. specifies an unbounded wait.  It is forbidden to specify a <em>timeout</em>

  738. when <em>blocking</em> is <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p>

  739. <p>The return value is <code class="docutils literal notranslate"><span class="pre">True</span></code> if the lock is acquired successfully,

  740. <code class="docutils literal notranslate"><span class="pre">False</span></code> if not (for example if the <em>timeout</em> expired).</p>

  741. <div class="versionchanged">

  742. <p><span class="versionmodified changed">Changed in version 3.2: </span>The <em>timeout</em> parameter is new.</p>

  743. </div>

  744. <div class="versionchanged">

  745. <p><span class="versionmodified changed">Changed in version 3.2: </span>Lock acquisition can now be interrupted by signals on POSIX if the

  746. underlying threading implementation supports it.</p>

  747. </div>

  748. </dd></dl>

  749. 

  750. <dl class="py method">

  751. <dt class="sig sig-object py" id="threading.Lock.release">

  752. <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="#threading.Lock.release" title="Link to this definition">¶</a></dt>

  753. <dd><p>Release a lock.  This can be called from any thread, not only the thread

  754. which has acquired the lock.</p>

  755. <p>When the lock is locked, reset it to unlocked, and return.  If any other threads

  756. are blocked waiting for the lock to become unlocked, allow exactly one of them

  757. to proceed.</p>

  758. <p>When invoked on an unlocked lock, a <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> is raised.</p>

  759. <p>There is no return value.</p>

  760. </dd></dl>

  761. 

  762. <dl class="py method">

  763. <dt class="sig sig-object py" id="threading.Lock.locked">

  764. <span class="sig-name descname"><span class="pre">locked</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Lock.locked" title="Link to this definition">¶</a></dt>

  765. <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the lock is acquired.</p>

  766. </dd></dl>

  767. 

  768. </dd></dl>

  769. 

  770. </section>

  771. <section id="rlock-objects">

  772. <span id="id3"></span><h2>RLock Objects<a class="headerlink" href="#rlock-objects" title="Link to this heading">¶</a></h2>

  773. <p>A reentrant lock is a synchronization primitive that may be acquired multiple

  774. times by the same thread.  Internally, it uses the concepts of “owning thread”

  775. and “recursion level” in addition to the locked/unlocked state used by primitive

  776. locks.  In the locked state, some thread owns the lock; in the unlocked state,

  777. no thread owns it.</p>

  778. <p>To lock the lock, a thread calls its <a class="reference internal" href="#threading.RLock.acquire" title="threading.RLock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> method; this

  779. returns once the thread owns the lock.  To unlock the lock, a thread calls

  780. its <a class="reference internal" href="#threading.Lock.release" title="threading.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> method. <a class="reference internal" href="#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a>/<a class="reference internal" href="#threading.Lock.release" title="threading.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a>

  781. call pairs may be nested; only the final <a class="reference internal" href="#threading.Lock.release" title="threading.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> (the

  782. <a class="reference internal" href="#threading.Lock.release" title="threading.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> of the outermost pair) resets the lock to unlocked and

  783. allows another thread blocked in <a class="reference internal" href="#threading.Lock.acquire" title="threading.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> to proceed.</p>

  784. <p>Reentrant locks also support the <a class="reference internal" href="#with-locks"><span class="std std-ref">context management protocol</span></a>.</p>

  785. <dl class="py class">

  786. <dt class="sig sig-object py" id="threading.RLock">

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

  788. <dd><p>This class implements reentrant lock objects.  A reentrant lock must be

  789. released by the thread that acquired it.  Once a thread has acquired a

  790. reentrant lock, the same thread may acquire it again without blocking; the

  791. thread must release it once for each time it has acquired it.</p>

  792. <p>Note that <code class="docutils literal notranslate"><span class="pre">RLock</span></code> is actually a factory function which returns an instance

  793. of the most efficient version of the concrete RLock class that is supported

  794. by the platform.</p>

  795. <dl class="py method">

  796. <dt class="sig sig-object py" id="threading.RLock.acquire">

  797. <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">blocking</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">-1</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#threading.RLock.acquire" title="Link to this definition">¶</a></dt>

  798. <dd><p>Acquire a lock, blocking or non-blocking.</p>

  799. <p>When invoked without arguments: if this thread already owns the lock, increment

  800. the recursion level by one, and return immediately.  Otherwise, if another

  801. thread owns the lock, block until the lock is unlocked.  Once the lock is

  802. unlocked (not owned by any thread), then grab ownership, set the recursion level

  803. to one, and return.  If more than one thread is blocked waiting until the lock

  804. is unlocked, only one at a time will be able to grab ownership of the lock.

  805. There is no return value in this case.</p>

  806. <p>When invoked with the <em>blocking</em> argument set to <code class="docutils literal notranslate"><span class="pre">True</span></code>, do the same thing as when

  807. called without arguments, and return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  808. <p>When invoked with the <em>blocking</em> argument set to <code class="docutils literal notranslate"><span class="pre">False</span></code>, do not block.  If a call

  809. without an argument would block, return <code class="docutils literal notranslate"><span class="pre">False</span></code> immediately; otherwise, do the

  810. same thing as when called without arguments, and return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  811. <p>When invoked with the floating-point <em>timeout</em> argument set to a positive

  812. value, block for at most the number of seconds specified by <em>timeout</em>

  813. and as long as the lock cannot be acquired.  Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the lock has

  814. been acquired, <code class="docutils literal notranslate"><span class="pre">False</span></code> if the timeout has elapsed.</p>

  815. <div class="versionchanged">

  816. <p><span class="versionmodified changed">Changed in version 3.2: </span>The <em>timeout</em> parameter is new.</p>

  817. </div>

  818. </dd></dl>

  819. 

  820. <dl class="py method">

  821. <dt class="sig sig-object py" id="threading.RLock.release">

  822. <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="#threading.RLock.release" title="Link to this definition">¶</a></dt>

  823. <dd><p>Release a lock, decrementing the recursion level.  If after the decrement it is

  824. zero, reset the lock to unlocked (not owned by any thread), and if any other

  825. threads are blocked waiting for the lock to become unlocked, allow exactly one

  826. of them to proceed.  If after the decrement the recursion level is still

  827. nonzero, the lock remains locked and owned by the calling thread.</p>

  828. <p>Only call this method when the calling thread owns the lock. A

  829. <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> is raised if this method is called when the lock is

  830. unlocked.</p>

  831. <p>There is no return value.</p>

  832. </dd></dl>

  833. 

  834. </dd></dl>

  835. 

  836. </section>

  837. <section id="condition-objects">

  838. <span id="id4"></span><h2>Condition Objects<a class="headerlink" href="#condition-objects" title="Link to this heading">¶</a></h2>

  839. <p>A condition variable is always associated with some kind of lock; this can be

  840. passed in or one will be created by default.  Passing one in is useful when

  841. several condition variables must share the same lock.  The lock is part of

  842. the condition object: you don’t have to track it separately.</p>

  843. <p>A condition variable obeys the <a class="reference internal" href="#with-locks"><span class="std std-ref">context management protocol</span></a>:

  844. using the <code class="docutils literal notranslate"><span class="pre">with</span></code> statement acquires the associated lock for the duration of

  845. the enclosed block.  The <a class="reference internal" href="#threading.Condition.acquire" title="threading.Condition.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> and

  846. <a class="reference internal" href="#threading.Condition.release" title="threading.Condition.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> methods also call the corresponding methods of

  847. the associated lock.</p>

  848. <p>Other methods must be called with the associated lock held.  The

  849. <a class="reference internal" href="#threading.Condition.wait" title="threading.Condition.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> method releases the lock, and then blocks until

  850. another thread awakens it by calling <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> or

  851. <a class="reference internal" href="#threading.Condition.notify_all" title="threading.Condition.notify_all"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify_all()</span></code></a>.  Once awakened, <a class="reference internal" href="#threading.Condition.wait" title="threading.Condition.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a>

  852. re-acquires the lock and returns.  It is also possible to specify a timeout.</p>

  853. <p>The <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> method wakes up one of the threads waiting for

  854. the condition variable, if any are waiting.  The <a class="reference internal" href="#threading.Condition.notify_all" title="threading.Condition.notify_all"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify_all()</span></code></a>

  855. method wakes up all threads waiting for the condition variable.</p>

  856. <p>Note: the <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> and <a class="reference internal" href="#threading.Condition.notify_all" title="threading.Condition.notify_all"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify_all()</span></code></a> methods

  857. don’t release the lock; this means that the thread or threads awakened will

  858. not return from their <a class="reference internal" href="#threading.Condition.wait" title="threading.Condition.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> call immediately, but only when

  859. the thread that called <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> or <a class="reference internal" href="#threading.Condition.notify_all" title="threading.Condition.notify_all"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify_all()</span></code></a>

  860. finally relinquishes ownership of the lock.</p>

  861. <p>The typical programming style using condition variables uses the lock to

  862. synchronize access to some shared state; threads that are interested in a

  863. particular change of state call <a class="reference internal" href="#threading.Condition.wait" title="threading.Condition.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> repeatedly until they

  864. see the desired state, while threads that modify the state call

  865. <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> or <a class="reference internal" href="#threading.Condition.notify_all" title="threading.Condition.notify_all"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify_all()</span></code></a> when they change

  866. the state in such a way that it could possibly be a desired state for one

  867. of the waiters.  For example, the following code is a generic

  868. producer-consumer situation with unlimited buffer capacity:</p>

  869. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Consume one item</span>

  870. <span class="k">with</span> <span class="n">cv</span><span class="p">:</span>

  871.     <span class="k">while</span> <span class="ow">not</span> <span class="n">an_item_is_available</span><span class="p">():</span>

  872.         <span class="n">cv</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>

  873.     <span class="n">get_an_available_item</span><span class="p">()</span>

  874. 

  875. <span class="c1"># Produce one item</span>

  876. <span class="k">with</span> <span class="n">cv</span><span class="p">:</span>

  877.     <span class="n">make_an_item_available</span><span class="p">()</span>

  878.     <span class="n">cv</span><span class="o">.</span><span class="n">notify</span><span class="p">()</span>

  879. </pre></div>

  880. </div>

  881. <p>The <code class="docutils literal notranslate"><span class="pre">while</span></code> loop checking for the application’s condition is necessary

  882. because <a class="reference internal" href="#threading.Condition.wait" title="threading.Condition.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> can return after an arbitrary long time,

  883. and the condition which prompted the <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> call may

  884. no longer hold true.  This is inherent to multi-threaded programming.  The

  885. <a class="reference internal" href="#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 can be used to automate the condition

  886. checking, and eases the computation of timeouts:</p>

  887. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Consume an item</span>

  888. <span class="k">with</span> <span class="n">cv</span><span class="p">:</span>

  889.     <span class="n">cv</span><span class="o">.</span><span class="n">wait_for</span><span class="p">(</span><span class="n">an_item_is_available</span><span class="p">)</span>

  890.     <span class="n">get_an_available_item</span><span class="p">()</span>

  891. </pre></div>

  892. </div>

  893. <p>To choose between <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> and <a class="reference internal" href="#threading.Condition.notify_all" title="threading.Condition.notify_all"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify_all()</span></code></a>,

  894. consider whether one state change can be interesting for only one or several

  895. waiting threads.  E.g. in a typical producer-consumer situation, adding one

  896. item to the buffer only needs to wake up one consumer thread.</p>

  897. <dl class="py class">

  898. <dt class="sig sig-object py" id="threading.Condition">

  899. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">Condition</span></span><span class="sig-paren">(</span><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">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#threading.Condition" title="Link to this definition">¶</a></dt>

  900. <dd><p>This class implements condition variable objects.  A condition variable

  901. allows one or more threads to wait until they are notified by another thread.</p>

  902. <p>If the <em>lock</em> argument is given and not <code class="docutils literal notranslate"><span class="pre">None</span></code>, it must be a <a class="reference internal" href="#threading.Lock" title="threading.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a>

  903. or <a class="reference internal" href="#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a> object, and it is used as the underlying lock.  Otherwise,

  904. a new <a class="reference internal" href="#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a> object is created and used as the underlying lock.</p>

  905. <div class="versionchanged">

  906. <p><span class="versionmodified changed">Changed in version 3.3: </span>changed from a factory function to a class.</p>

  907. </div>

  908. <dl class="py method">

  909. <dt class="sig sig-object py" id="threading.Condition.acquire">

  910. <span class="sig-name descname"><span class="pre">acquire</span></span><span class="sig-paren">(</span><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="#threading.Condition.acquire" title="Link to this definition">¶</a></dt>

  911. <dd><p>Acquire the underlying lock. This method calls the corresponding method on

  912. the underlying lock; the return value is whatever that method returns.</p>

  913. </dd></dl>

  914. 

  915. <dl class="py method">

  916. <dt class="sig sig-object py" id="threading.Condition.release">

  917. <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="#threading.Condition.release" title="Link to this definition">¶</a></dt>

  918. <dd><p>Release the underlying lock. This method calls the corresponding method on

  919. the underlying lock; there is no return value.</p>

  920. </dd></dl>

  921. 

  922. <dl class="py method">

  923. <dt class="sig sig-object py" id="threading.Condition.wait">

  924. <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">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="#threading.Condition.wait" title="Link to this definition">¶</a></dt>

  925. <dd><p>Wait until notified or until a timeout occurs. If the calling thread has

  926. not acquired the lock when this method is called, a <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> is

  927. raised.</p>

  928. <p>This method releases the underlying lock, and then blocks until it is

  929. awakened by a <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> or <a class="reference internal" href="#threading.Condition.notify_all" title="threading.Condition.notify_all"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify_all()</span></code></a> call for the same

  930. condition variable in another thread, or until the optional timeout

  931. occurs.  Once awakened or timed out, it re-acquires the lock and returns.</p>

  932. <p>When the <em>timeout</em> argument is present and not <code class="docutils literal notranslate"><span class="pre">None</span></code>, it should be a

  933. floating point number specifying a timeout for the operation in seconds

  934. (or fractions thereof).</p>

  935. <p>When the underlying lock is an <a class="reference internal" href="#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a>, it is not released using

  936. its <a class="reference internal" href="#threading.Condition.release" title="threading.Condition.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> method, since this may not actually unlock the lock

  937. when it was acquired multiple times recursively.  Instead, an internal

  938. interface of the <a class="reference internal" href="#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a> class is used, which really unlocks it

  939. even when it has been recursively acquired several times. Another internal

  940. interface is then used to restore the recursion level when the lock is

  941. reacquired.</p>

  942. <p>The return value is <code class="docutils literal notranslate"><span class="pre">True</span></code> unless a given <em>timeout</em> expired, in which

  943. case it is <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p>

  944. <div class="versionchanged">

  945. <p><span class="versionmodified changed">Changed in version 3.2: </span>Previously, the method always returned <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>

  946. </div>

  947. </dd></dl>

  948. 

  949. <dl class="py method">

  950. <dt class="sig sig-object py" id="threading.Condition.wait_for">

  951. <span class="sig-name descname"><span class="pre">wait_for</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">predicate</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="#threading.Condition.wait_for" title="Link to this definition">¶</a></dt>

  952. <dd><p>Wait until a condition evaluates to true.  <em>predicate</em> should be a

  953. callable which result will be interpreted as a boolean value.

  954. A <em>timeout</em> may be provided giving the maximum time to wait.</p>

  955. <p>This utility method may call <a class="reference internal" href="#threading.Condition.wait" title="threading.Condition.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> repeatedly until the predicate

  956. is satisfied, or until a timeout occurs. The return value is

  957. the last return value of the predicate and will evaluate to

  958. <code class="docutils literal notranslate"><span class="pre">False</span></code> if the method timed out.</p>

  959. <p>Ignoring the timeout feature, calling this method is roughly equivalent to

  960. writing:</p>

  961. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="ow">not</span> <span class="n">predicate</span><span class="p">():</span>

  962.     <span class="n">cv</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>

  963. </pre></div>

  964. </div>

  965. <p>Therefore, the same rules apply as with <a class="reference internal" href="#threading.Condition.wait" title="threading.Condition.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a>: The lock must be

  966. held when called and is re-acquired on return.  The predicate is evaluated

  967. with the lock held.</p>

  968. <div class="versionadded">

  969. <p><span class="versionmodified added">New in version 3.2.</span></p>

  970. </div>

  971. </dd></dl>

  972. 

  973. <dl class="py method">

  974. <dt class="sig sig-object py" id="threading.Condition.notify">

  975. <span class="sig-name descname"><span class="pre">notify</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">n</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">1</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#threading.Condition.notify" title="Link to this definition">¶</a></dt>

  976. <dd><p>By default, wake up one thread waiting on this condition, if any.  If the

  977. calling thread has not acquired the lock when this method is called, a

  978. <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> is raised.</p>

  979. <p>This method wakes up at most <em>n</em> of the threads waiting for the condition

  980. variable; it is a no-op if no threads are waiting.</p>

  981. <p>The current implementation wakes up exactly <em>n</em> threads, if at least <em>n</em>

  982. threads are waiting.  However, it’s not safe to rely on this behavior.

  983. A future, optimized implementation may occasionally wake up more than

  984. <em>n</em> threads.</p>

  985. <p>Note: an awakened thread does not actually return from its <a class="reference internal" href="#threading.Condition.wait" title="threading.Condition.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a>

  986. call until it can reacquire the lock.  Since <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> does not

  987. release the lock, its caller should.</p>

  988. </dd></dl>

  989. 

  990. <dl class="py method">

  991. <dt class="sig sig-object py" id="threading.Condition.notify_all">

  992. <span class="sig-name descname"><span class="pre">notify_all</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Condition.notify_all" title="Link to this definition">¶</a></dt>

  993. <dd><p>Wake up all threads waiting on this condition.  This method acts like

  994. <a class="reference internal" href="#threading.Condition.notify" title="threading.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a>, but wakes up all waiting threads instead of one. If the

  995. calling thread has not acquired the lock when this method is called, a

  996. <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> is raised.</p>

  997. <p>The method <code class="docutils literal notranslate"><span class="pre">notifyAll</span></code> is a deprecated alias for this method.</p>

  998. </dd></dl>

  999. 

  1000. </dd></dl>

  1001. 

  1002. </section>

  1003. <section id="semaphore-objects">

  1004. <span id="id5"></span><h2>Semaphore Objects<a class="headerlink" href="#semaphore-objects" title="Link to this heading">¶</a></h2>

  1005. <p>This is one of the oldest synchronization primitives in the history of computer

  1006. science, invented by the early Dutch computer scientist Edsger W. Dijkstra (he

  1007. used the names <code class="docutils literal notranslate"><span class="pre">P()</span></code> and <code class="docutils literal notranslate"><span class="pre">V()</span></code> instead of <a class="reference internal" href="#threading.Semaphore.acquire" title="threading.Semaphore.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> and

  1008. <a class="reference internal" href="#threading.Semaphore.release" title="threading.Semaphore.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a>).</p>

  1009. <p>A semaphore manages an internal counter which is decremented by each

  1010. <a class="reference internal" href="#threading.Semaphore.acquire" title="threading.Semaphore.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> call and incremented by each <a class="reference internal" href="#threading.Semaphore.release" title="threading.Semaphore.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a>

  1011. call.  The counter can never go below zero; when <a class="reference internal" href="#threading.Semaphore.acquire" title="threading.Semaphore.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a>

  1012. finds that it is zero, it blocks, waiting until some other thread calls

  1013. <a class="reference internal" href="#threading.Semaphore.release" title="threading.Semaphore.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a>.</p>

  1014. <p>Semaphores also support the <a class="reference internal" href="#with-locks"><span class="std std-ref">context management protocol</span></a>.</p>

  1015. <dl class="py class">

  1016. <dt class="sig sig-object py" id="threading.Semaphore">

  1017. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">Semaphore</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">value</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">1</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#threading.Semaphore" title="Link to this definition">¶</a></dt>

  1018. <dd><p>This class implements semaphore objects.  A semaphore manages an atomic

  1019. counter representing the number of <a class="reference internal" href="#threading.Semaphore.release" title="threading.Semaphore.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> calls minus the number of

  1020. <a class="reference internal" href="#threading.Semaphore.acquire" title="threading.Semaphore.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> calls, plus an initial value.  The <a class="reference internal" href="#threading.Semaphore.acquire" title="threading.Semaphore.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a> method

  1021. blocks if necessary until it can return without making the counter negative.

  1022. If not given, <em>value</em> defaults to 1.</p>

  1023. <p>The optional argument gives the initial <em>value</em> for the internal counter; it

  1024. defaults to <code class="docutils literal notranslate"><span class="pre">1</span></code>. If the <em>value</em> given is less than 0, <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

  1025. raised.</p>

  1026. <div class="versionchanged">

  1027. <p><span class="versionmodified changed">Changed in version 3.3: </span>changed from a factory function to a class.</p>

  1028. </div>

  1029. <dl class="py method">

  1030. <dt class="sig sig-object py" id="threading.Semaphore.acquire">

  1031. <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">blocking</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="#threading.Semaphore.acquire" title="Link to this definition">¶</a></dt>

  1032. <dd><p>Acquire a semaphore.</p>

  1033. <p>When invoked without arguments:</p>

  1034. <ul class="simple">

  1035. <li><p>If the internal counter is larger than zero on entry, decrement it by

  1036. one and return <code class="docutils literal notranslate"><span class="pre">True</span></code> immediately.</p></li>

  1037. <li><p>If the internal counter is zero on entry, block until awoken by a call to

  1038. <a class="reference internal" href="#threading.Semaphore.release" title="threading.Semaphore.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a>.  Once awoken (and the counter is greater

  1039. than 0), decrement the counter by 1 and return <code class="docutils literal notranslate"><span class="pre">True</span></code>.  Exactly one

  1040. thread will be awoken by each call to <a class="reference internal" href="#threading.Semaphore.release" title="threading.Semaphore.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a>.  The

  1041. order in which threads are awoken should not be relied on.</p></li>

  1042. </ul>

  1043. <p>When invoked with <em>blocking</em> set to <code class="docutils literal notranslate"><span class="pre">False</span></code>, do not block.  If a call

  1044. without an argument would block, return <code class="docutils literal notranslate"><span class="pre">False</span></code> immediately; otherwise, do

  1045. the same thing as when called without arguments, and return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  1046. <p>When invoked with a <em>timeout</em> other than <code class="docutils literal notranslate"><span class="pre">None</span></code>, it will block for at

  1047. most <em>timeout</em> seconds.  If acquire does not complete successfully in

  1048. that interval, return <code class="docutils literal notranslate"><span class="pre">False</span></code>.  Return <code class="docutils literal notranslate"><span class="pre">True</span></code> otherwise.</p>

  1049. <div class="versionchanged">

  1050. <p><span class="versionmodified changed">Changed in version 3.2: </span>The <em>timeout</em> parameter is new.</p>

  1051. </div>

  1052. </dd></dl>

  1053. 

  1054. <dl class="py method">

  1055. <dt class="sig sig-object py" id="threading.Semaphore.release">

  1056. <span class="sig-name descname"><span class="pre">release</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">n</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">1</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#threading.Semaphore.release" title="Link to this definition">¶</a></dt>

  1057. <dd><p>Release a semaphore, incrementing the internal counter by <em>n</em>.  When it

  1058. was zero on entry and other threads are waiting for it to become larger

  1059. than zero again, wake up <em>n</em> of those threads.</p>

  1060. <div class="versionchanged">

  1061. <p><span class="versionmodified changed">Changed in version 3.9: </span>Added the <em>n</em> parameter to release multiple waiting threads at once.</p>

  1062. </div>

  1063. </dd></dl>

  1064. 

  1065. </dd></dl>

  1066. 

  1067. <dl class="py class">

  1068. <dt class="sig sig-object py" id="threading.BoundedSemaphore">

  1069. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">BoundedSemaphore</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">value</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">1</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#threading.BoundedSemaphore" title="Link to this definition">¶</a></dt>

  1070. <dd><p>Class implementing bounded semaphore objects.  A bounded semaphore checks to

  1071. make sure its current value doesn’t exceed its initial value.  If it does,

  1072. <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. In most situations semaphores are used to guard

  1073. resources with limited capacity.  If the semaphore is released too many times

  1074. it’s a sign of a bug.  If not given, <em>value</em> defaults to 1.</p>

  1075. <div class="versionchanged">

  1076. <p><span class="versionmodified changed">Changed in version 3.3: </span>changed from a factory function to a class.</p>

  1077. </div>

  1078. </dd></dl>

  1079. 

  1080. <section id="semaphore-example">

  1081. <span id="semaphore-examples"></span><h3><a class="reference internal" href="#threading.Semaphore" title="threading.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">Semaphore</span></code></a> Example<a class="headerlink" href="#semaphore-example" title="Link to this heading">¶</a></h3>

  1082. <p>Semaphores are often used to guard resources with limited capacity, for example,

  1083. a database server.  In any situation where the size of the resource is fixed,

  1084. you should use a bounded semaphore.  Before spawning any worker threads, your

  1085. main thread would initialize the semaphore:</p>

  1086. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">maxconnections</span> <span class="o">=</span> <span class="mi">5</span>

  1087. <span class="c1"># ...</span>

  1088. <span class="n">pool_sema</span> <span class="o">=</span> <span class="n">BoundedSemaphore</span><span class="p">(</span><span class="n">value</span><span class="o">=</span><span class="n">maxconnections</span><span class="p">)</span>

  1089. </pre></div>

  1090. </div>

  1091. <p>Once spawned, worker threads call the semaphore’s acquire and release methods

  1092. when they need to connect to the server:</p>

  1093. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">pool_sema</span><span class="p">:</span>

  1094.     <span class="n">conn</span> <span class="o">=</span> <span class="n">connectdb</span><span class="p">()</span>

  1095.     <span class="k">try</span><span class="p">:</span>

  1096.         <span class="c1"># ... use connection ...</span>

  1097.     <span class="k">finally</span><span class="p">:</span>

  1098.         <span class="n">conn</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>

  1099. </pre></div>

  1100. </div>

  1101. <p>The use of a bounded semaphore reduces the chance that a programming error which

  1102. causes the semaphore to be released more than it’s acquired will go undetected.</p>

  1103. </section>

  1104. </section>

  1105. <section id="event-objects">

  1106. <span id="id6"></span><h2>Event Objects<a class="headerlink" href="#event-objects" title="Link to this heading">¶</a></h2>

  1107. <p>This is one of the simplest mechanisms for communication between threads: one

  1108. thread signals an event and other threads wait for it.</p>

  1109. <p>An event object manages an internal flag that can be set to true with the

  1110. <a class="reference internal" href="#threading.Event.set" title="threading.Event.set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set()</span></code></a> method and reset to false with the <a class="reference internal" href="#threading.Event.clear" title="threading.Event.clear"><code class="xref py py-meth docutils literal notranslate"><span class="pre">clear()</span></code></a>

  1111. method.  The <a class="reference internal" href="#threading.Event.wait" title="threading.Event.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> method blocks until the flag is true.</p>

  1112. <dl class="py class">

  1113. <dt class="sig sig-object py" id="threading.Event">

  1114. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">Event</span></span><a class="headerlink" href="#threading.Event" title="Link to this definition">¶</a></dt>

  1115. <dd><p>Class implementing event objects.  An event manages a flag that can be set to

  1116. true with the <a class="reference internal" href="#threading.Event.set" title="threading.Event.set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set()</span></code></a> method and reset to false with the

  1117. <a class="reference internal" href="#threading.Event.clear" title="threading.Event.clear"><code class="xref py py-meth docutils literal notranslate"><span class="pre">clear()</span></code></a> method.  The <a class="reference internal" href="#threading.Event.wait" title="threading.Event.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> method blocks until the flag is true.

  1118. The flag is initially false.</p>

  1119. <div class="versionchanged">

  1120. <p><span class="versionmodified changed">Changed in version 3.3: </span>changed from a factory function to a class.</p>

  1121. </div>

  1122. <dl class="py method">

  1123. <dt class="sig sig-object py" id="threading.Event.is_set">

  1124. <span class="sig-name descname"><span class="pre">is_set</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Event.is_set" title="Link to this definition">¶</a></dt>

  1125. <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if and only if the internal flag is true.</p>

  1126. <p>The method <code class="docutils literal notranslate"><span class="pre">isSet</span></code> is a deprecated alias for this method.</p>

  1127. </dd></dl>

  1128. 

  1129. <dl class="py method">

  1130. <dt class="sig sig-object py" id="threading.Event.set">

  1131. <span class="sig-name descname"><span class="pre">set</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Event.set" title="Link to this definition">¶</a></dt>

  1132. <dd><p>Set the internal flag to true. All threads waiting for it to become true

  1133. are awakened. Threads that call <a class="reference internal" href="#threading.Event.wait" title="threading.Event.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> once the flag is true will

  1134. not block at all.</p>

  1135. </dd></dl>

  1136. 

  1137. <dl class="py method">

  1138. <dt class="sig sig-object py" id="threading.Event.clear">

  1139. <span class="sig-name descname"><span class="pre">clear</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Event.clear" title="Link to this definition">¶</a></dt>

  1140. <dd><p>Reset the internal flag to false. Subsequently, threads calling

  1141. <a class="reference internal" href="#threading.Event.wait" title="threading.Event.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> will block until <a class="reference internal" href="#threading.Event.set" title="threading.Event.set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set()</span></code></a> is called to set the internal

  1142. flag to true again.</p>

  1143. </dd></dl>

  1144. 

  1145. <dl class="py method">

  1146. <dt class="sig sig-object py" id="threading.Event.wait">

  1147. <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">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="#threading.Event.wait" title="Link to this definition">¶</a></dt>

  1148. <dd><p>Block as long as the internal flag is false and the timeout, if given,

  1149. has not expired. The return value represents the

  1150. reason that this blocking method returned; <code class="docutils literal notranslate"><span class="pre">True</span></code> if returning because

  1151. the internal flag is set to true, or <code class="docutils literal notranslate"><span class="pre">False</span></code> if a timeout is given and

  1152. the the internal flag did not become true within the given wait time.</p>

  1153. <p>When the timeout argument is present and not <code class="docutils literal notranslate"><span class="pre">None</span></code>, it should be a

  1154. floating point number specifying a timeout for the operation in seconds,

  1155. or fractions thereof.</p>

  1156. <div class="versionchanged">

  1157. <p><span class="versionmodified changed">Changed in version 3.1: </span>Previously, the method always returned <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>

  1158. </div>

  1159. </dd></dl>

  1160. 

  1161. </dd></dl>

  1162. 

  1163. </section>

  1164. <section id="timer-objects">

  1165. <span id="id7"></span><h2>Timer Objects<a class="headerlink" href="#timer-objects" title="Link to this heading">¶</a></h2>

  1166. <p>This class represents an action that should be run only after a certain amount

  1167. of time has passed — a timer.  <a class="reference internal" href="#threading.Timer" title="threading.Timer"><code class="xref py py-class docutils literal notranslate"><span class="pre">Timer</span></code></a> is a subclass of <a class="reference internal" href="#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">Thread</span></code></a>

  1168. and as such also functions as an example of creating custom threads.</p>

  1169. <p>Timers are started, as with threads, by calling their <a class="reference internal" href="#threading.Thread.start" title="threading.Thread.start"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Timer.start</span></code></a>

  1170. method.  The timer can be stopped (before its action has begun) by calling the

  1171. <a class="reference internal" href="#threading.Timer.cancel" title="threading.Timer.cancel"><code class="xref py py-meth docutils literal notranslate"><span class="pre">cancel()</span></code></a> method.  The interval the timer will wait before

  1172. executing its action may not be exactly the same as the interval specified by

  1173. the user.</p>

  1174. <p>For example:</p>

  1175. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">hello</span><span class="p">():</span>

  1176.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;hello, world&quot;</span><span class="p">)</span>

  1177. 

  1178. <span class="n">t</span> <span class="o">=</span> <span class="n">Timer</span><span class="p">(</span><span class="mf">30.0</span><span class="p">,</span> <span class="n">hello</span><span class="p">)</span>

  1179. <span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>  <span class="c1"># after 30 seconds, &quot;hello, world&quot; will be printed</span>

  1180. </pre></div>

  1181. </div>

  1182. <dl class="py class">

  1183. <dt class="sig sig-object py" id="threading.Timer">

  1184. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">Timer</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">interval</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">function</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">None</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">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#threading.Timer" title="Link to this definition">¶</a></dt>

  1185. <dd><p>Create a timer that will run <em>function</em> with arguments <em>args</em> and  keyword

  1186. arguments <em>kwargs</em>, after <em>interval</em> seconds have passed.

  1187. If <em>args</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> (the default) then an empty list will be used.

  1188. If <em>kwargs</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> (the default) then an empty dict will be used.</p>

  1189. <div class="versionchanged">

  1190. <p><span class="versionmodified changed">Changed in version 3.3: </span>changed from a factory function to a class.</p>

  1191. </div>

  1192. <dl class="py method">

  1193. <dt class="sig sig-object py" id="threading.Timer.cancel">

  1194. <span class="sig-name descname"><span class="pre">cancel</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Timer.cancel" title="Link to this definition">¶</a></dt>

  1195. <dd><p>Stop the timer, and cancel the execution of the timer’s action.  This will

  1196. only work if the timer is still in its waiting stage.</p>

  1197. </dd></dl>

  1198. 

  1199. </dd></dl>

  1200. 

  1201. </section>

  1202. <section id="barrier-objects">

  1203. <h2>Barrier Objects<a class="headerlink" href="#barrier-objects" title="Link to this heading">¶</a></h2>

  1204. <div class="versionadded">

  1205. <p><span class="versionmodified added">New in version 3.2.</span></p>

  1206. </div>

  1207. <p>This class provides a simple synchronization primitive for use by a fixed number

  1208. of threads that need to wait for each other.  Each of the threads tries to pass

  1209. the barrier by calling the <a class="reference internal" href="#threading.Barrier.wait" title="threading.Barrier.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> method and will block until

  1210. all of the threads have made their <a class="reference internal" href="#threading.Barrier.wait" title="threading.Barrier.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> calls. At this point,

  1211. the threads are released simultaneously.</p>

  1212. <p>The barrier can be reused any number of times for the same number of threads.</p>

  1213. <p>As an example, here is a simple way to synchronize a client and server thread:</p>

  1214. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">b</span> <span class="o">=</span> <span class="n">Barrier</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="n">timeout</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span>

  1215. 

  1216. <span class="k">def</span> <span class="nf">server</span><span class="p">():</span>

  1217.     <span class="n">start_server</span><span class="p">()</span>

  1218.     <span class="n">b</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>

  1219.     <span class="k">while</span> <span class="kc">True</span><span class="p">:</span>

  1220.         <span class="n">connection</span> <span class="o">=</span> <span class="n">accept_connection</span><span class="p">()</span>

  1221.         <span class="n">process_server_connection</span><span class="p">(</span><span class="n">connection</span><span class="p">)</span>

  1222. 

  1223. <span class="k">def</span> <span class="nf">client</span><span class="p">():</span>

  1224.     <span class="n">b</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>

  1225.     <span class="k">while</span> <span class="kc">True</span><span class="p">:</span>

  1226.         <span class="n">connection</span> <span class="o">=</span> <span class="n">make_connection</span><span class="p">()</span>

  1227.         <span class="n">process_client_connection</span><span class="p">(</span><span class="n">connection</span><span class="p">)</span>

  1228. </pre></div>

  1229. </div>

  1230. <dl class="py class">

  1231. <dt class="sig sig-object py" id="threading.Barrier">

  1232. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">threading.</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>, <em class="sig-param"><span class="n"><span class="pre">action</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">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="#threading.Barrier" title="Link to this definition">¶</a></dt>

  1233. <dd><p>Create a barrier object for <em>parties</em> number of threads.  An <em>action</em>, when

  1234. provided, is a callable to be called by one of the threads when they are

  1235. released.  <em>timeout</em> is the default timeout value if none is specified for

  1236. the <a class="reference internal" href="#threading.Barrier.wait" title="threading.Barrier.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> method.</p>

  1237. <dl class="py method">

  1238. <dt class="sig sig-object py" id="threading.Barrier.wait">

  1239. <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">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="#threading.Barrier.wait" title="Link to this definition">¶</a></dt>

  1240. <dd><p>Pass the barrier.  When all the threads party to the barrier have called

  1241. this function, they are all released simultaneously.  If a <em>timeout</em> is

  1242. provided, it is used in preference to any that was supplied to the class

  1243. constructor.</p>

  1244. <p>The return value is an integer in the range 0 to <em>parties</em> – 1, different

  1245. for each thread.  This can be used to select a thread to do some special

  1246. housekeeping, e.g.:</p>

  1247. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">i</span> <span class="o">=</span> <span class="n">barrier</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>

  1248. <span class="k">if</span> <span class="n">i</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>

  1249.     <span class="c1"># Only one thread needs to print this</span>

  1250.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;passed the barrier&quot;</span><span class="p">)</span>

  1251. </pre></div>

  1252. </div>

  1253. <p>If an <em>action</em> was provided to the constructor, one of the threads will

  1254. have called it prior to being released.  Should this call raise an error,

  1255. the barrier is put into the broken state.</p>

  1256. <p>If the call times out, the barrier is put into the broken state.</p>

  1257. <p>This method may raise a <a class="reference internal" href="#threading.BrokenBarrierError" title="threading.BrokenBarrierError"><code class="xref py py-class docutils literal notranslate"><span class="pre">BrokenBarrierError</span></code></a> exception if the

  1258. barrier is broken or reset while a thread is waiting.</p>

  1259. </dd></dl>

  1260. 

  1261. <dl class="py method">

  1262. <dt class="sig sig-object py" id="threading.Barrier.reset">

  1263. <span class="sig-name descname"><span class="pre">reset</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Barrier.reset" title="Link to this definition">¶</a></dt>

  1264. <dd><p>Return the barrier to the default, empty state.  Any threads waiting on it

  1265. will receive the <a class="reference internal" href="#threading.BrokenBarrierError" title="threading.BrokenBarrierError"><code class="xref py py-class docutils literal notranslate"><span class="pre">BrokenBarrierError</span></code></a> exception.</p>

  1266. <p>Note that using this function may require some external

  1267. synchronization if there are other threads whose state is unknown.  If a

  1268. barrier is broken it may be better to just leave it and create a new one.</p>

  1269. </dd></dl>

  1270. 

  1271. <dl class="py method">

  1272. <dt class="sig sig-object py" id="threading.Barrier.abort">

  1273. <span class="sig-name descname"><span class="pre">abort</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#threading.Barrier.abort" title="Link to this definition">¶</a></dt>

  1274. <dd><p>Put the barrier into a broken state.  This causes any active or future

  1275. calls to <a class="reference internal" href="#threading.Barrier.wait" title="threading.Barrier.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> to fail with the <a class="reference internal" href="#threading.BrokenBarrierError" title="threading.BrokenBarrierError"><code class="xref py py-class docutils literal notranslate"><span class="pre">BrokenBarrierError</span></code></a>.  Use

  1276. this for example if one of the threads needs to abort, to avoid deadlocking the

  1277. application.</p>

  1278. <p>It may be preferable to simply create the barrier with a sensible

  1279. <em>timeout</em> value to automatically guard against one of the threads going

  1280. awry.</p>

  1281. </dd></dl>

  1282. 

  1283. <dl class="py attribute">

  1284. <dt class="sig sig-object py" id="threading.Barrier.parties">

  1285. <span class="sig-name descname"><span class="pre">parties</span></span><a class="headerlink" href="#threading.Barrier.parties" title="Link to this definition">¶</a></dt>

  1286. <dd><p>The number of threads required to pass the barrier.</p>

  1287. </dd></dl>

  1288. 

  1289. <dl class="py attribute">

  1290. <dt class="sig sig-object py" id="threading.Barrier.n_waiting">

  1291. <span class="sig-name descname"><span class="pre">n_waiting</span></span><a class="headerlink" href="#threading.Barrier.n_waiting" title="Link to this definition">¶</a></dt>

  1292. <dd><p>The number of threads currently waiting in the barrier.</p>

  1293. </dd></dl>

  1294. 

  1295. <dl class="py attribute">

  1296. <dt class="sig sig-object py" id="threading.Barrier.broken">

  1297. <span class="sig-name descname"><span class="pre">broken</span></span><a class="headerlink" href="#threading.Barrier.broken" title="Link to this definition">¶</a></dt>

  1298. <dd><p>A boolean that is <code class="docutils literal notranslate"><span class="pre">True</span></code> if the barrier is in the broken state.</p>

  1299. </dd></dl>

  1300. 

  1301. </dd></dl>

  1302. 

  1303. <dl class="py exception">

  1304. <dt class="sig sig-object py" id="threading.BrokenBarrierError">

  1305. <em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">threading.</span></span><span class="sig-name descname"><span class="pre">BrokenBarrierError</span></span><a class="headerlink" href="#threading.BrokenBarrierError" title="Link to this definition">¶</a></dt>

  1306. <dd><p>This exception, a subclass of <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>, is raised when the

  1307. <a class="reference internal" href="#threading.Barrier" title="threading.Barrier"><code class="xref py py-class docutils literal notranslate"><span class="pre">Barrier</span></code></a> object is reset or broken.</p>

  1308. </dd></dl>

  1309. 

  1310. </section>

  1311. <section id="using-locks-conditions-and-semaphores-in-the-with-statement">

  1312. <span id="with-locks"></span><h2>Using locks, conditions, and semaphores in the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement<a class="headerlink" href="#using-locks-conditions-and-semaphores-in-the-with-statement" title="Link to this heading">¶</a></h2>

  1313. <p>All of the objects provided by this module that have <code class="docutils literal notranslate"><span class="pre">acquire</span></code> and

  1314. <code class="docutils literal notranslate"><span class="pre">release</span></code> methods can be used as context managers for a <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>

  1315. statement.  The <code class="docutils literal notranslate"><span class="pre">acquire</span></code> method will be called when the block is

  1316. entered, and <code class="docutils literal notranslate"><span class="pre">release</span></code> will be called when the block is exited.  Hence,

  1317. the following snippet:</p>

  1318. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">some_lock</span><span class="p">:</span>

  1319.     <span class="c1"># do something...</span>

  1320. </pre></div>

  1321. </div>

  1322. <p>is equivalent to:</p>

  1323. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">some_lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>

  1324. <span class="k">try</span><span class="p">:</span>

  1325.     <span class="c1"># do something...</span>

  1326. <span class="k">finally</span><span class="p">:</span>

  1327.     <span class="n">some_lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>

  1328. </pre></div>

  1329. </div>

  1330. <p>Currently, <a class="reference internal" href="#threading.Lock" title="threading.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a>, <a class="reference internal" href="#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">RLock</span></code></a>, <a class="reference internal" href="#threading.Condition" title="threading.Condition"><code class="xref py py-class docutils literal notranslate"><span class="pre">Condition</span></code></a>,

  1331. <a class="reference internal" href="#threading.Semaphore" title="threading.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">Semaphore</span></code></a>, and <a class="reference internal" href="#threading.BoundedSemaphore" title="threading.BoundedSemaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">BoundedSemaphore</span></code></a> objects may be used as

  1332. <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> statement context managers.</p>

  1333. </section>

  1334. </section>

  1335. 

  1336. 

  1337.             <div class="clearer"></div>

  1338.           </div>

  1339.         </div>

  1340.       </div>

  1341.       <div class="sphinxsidebar" role="navigation" aria-label="main navigation">

  1342.         <div class="sphinxsidebarwrapper">

  1343.   <div>

  1344.     <h3><a href="../contents.html">Table of Contents</a></h3>

  1345.     <ul>

  1346. <li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code> — Thread-based parallelism</a><ul>

  1347. <li><a class="reference internal" href="#thread-local-data">Thread-Local Data</a></li>

  1348. <li><a class="reference internal" href="#thread-objects">Thread Objects</a></li>

  1349. <li><a class="reference internal" href="#lock-objects">Lock Objects</a></li>

  1350. <li><a class="reference internal" href="#rlock-objects">RLock Objects</a></li>

  1351. <li><a class="reference internal" href="#condition-objects">Condition Objects</a></li>

  1352. <li><a class="reference internal" href="#semaphore-objects">Semaphore Objects</a><ul>

  1353. <li><a class="reference internal" href="#semaphore-example"><code class="xref py py-class docutils literal notranslate"><span class="pre">Semaphore</span></code> Example</a></li>

  1354. </ul>

  1355. </li>

  1356. <li><a class="reference internal" href="#event-objects">Event Objects</a></li>

  1357. <li><a class="reference internal" href="#timer-objects">Timer Objects</a></li>

  1358. <li><a class="reference internal" href="#barrier-objects">Barrier Objects</a></li>

  1359. <li><a class="reference internal" href="#using-locks-conditions-and-semaphores-in-the-with-statement">Using locks, conditions, and semaphores in the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement</a></li>

  1360. </ul>

  1361. </li>

  1362. </ul>

  1363. 

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  1368.                           title="previous chapter">Concurrent Execution</a></p>

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  1372.     <p class="topless"><a href="multiprocessing.html"

  1373.                           title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code> — Process-based parallelism</a></p>

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