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  198.   <section id="synchronization-primitives">

  199. <span id="asyncio-sync"></span><h1>Synchronization Primitives<a class="headerlink" href="#synchronization-primitives" title="Link to this heading">¶</a></h1>

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

  201. <hr class="docutils" />

  202. <p>asyncio synchronization primitives are designed to be similar to

  203. those of the <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module with two important caveats:</p>

  204. <ul class="simple">

  205. <li><p>asyncio primitives are not thread-safe, therefore they should not

  206. be used for OS thread synchronization (use <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> for

  207. that);</p></li>

  208. <li><p>methods of these synchronization primitives do not accept the <em>timeout</em>

  209. argument; use the <a class="reference internal" href="asyncio-task.html#asyncio.wait_for" title="asyncio.wait_for"><code class="xref py py-func docutils literal notranslate"><span class="pre">asyncio.wait_for()</span></code></a> function to perform

  210. operations with timeouts.</p></li>

  211. </ul>

  212. <p>asyncio has the following basic synchronization primitives:</p>

  213. <ul class="simple">

  214. <li><p><a class="reference internal" href="#asyncio.Lock" title="asyncio.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a></p></li>

  215. <li><p><a class="reference internal" href="#asyncio.Event" title="asyncio.Event"><code class="xref py py-class docutils literal notranslate"><span class="pre">Event</span></code></a></p></li>

  216. <li><p><a class="reference internal" href="#asyncio.Condition" title="asyncio.Condition"><code class="xref py py-class docutils literal notranslate"><span class="pre">Condition</span></code></a></p></li>

  217. <li><p><a class="reference internal" href="#asyncio.Semaphore" title="asyncio.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">Semaphore</span></code></a></p></li>

  218. <li><p><a class="reference internal" href="#asyncio.BoundedSemaphore" title="asyncio.BoundedSemaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">BoundedSemaphore</span></code></a></p></li>

  219. <li><p><a class="reference internal" href="#asyncio.Barrier" title="asyncio.Barrier"><code class="xref py py-class docutils literal notranslate"><span class="pre">Barrier</span></code></a></p></li>

  220. </ul>

  221. <hr class="docutils" />

  222. <section id="lock">

  223. <h2>Lock<a class="headerlink" href="#lock" title="Link to this heading">¶</a></h2>

  224. <dl class="py class">

  225. <dt class="sig sig-object py" id="asyncio.Lock">

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

  227. <dd><p>Implements a mutex lock for asyncio tasks.  Not thread-safe.</p>

  228. <p>An asyncio lock can be used to guarantee exclusive access to a

  229. shared resource.</p>

  230. <p>The preferred way to use a Lock is an <a class="reference internal" href="../reference/compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a>

  231. statement:</p>

  232. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">lock</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>

  233. 

  234. <span class="c1"># ... later</span>

  235. <span class="k">async</span> <span class="k">with</span> <span class="n">lock</span><span class="p">:</span>

  236.     <span class="c1"># access shared state</span>

  237. </pre></div>

  238. </div>

  239. <p>which is equivalent to:</p>

  240. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">lock</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>

  241. 

  242. <span class="c1"># ... later</span>

  243. <span class="k">await</span> <span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>

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

  245.     <span class="c1"># access shared state</span>

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

  247.     <span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>

  248. </pre></div>

  249. </div>

  250. <div class="versionchanged">

  251. <p><span class="versionmodified changed">Changed in version 3.10: </span>Removed the <em>loop</em> parameter.</p>

  252. </div>

  253. <dl class="py method">

  254. <dt class="sig sig-object py" id="asyncio.Lock.acquire">

  255. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">acquire</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Lock.acquire" title="Link to this definition">¶</a></dt>

  256. <dd><p>Acquire the lock.</p>

  257. <p>This method waits until the lock is <em>unlocked</em>, sets it to

  258. <em>locked</em> and returns <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  259. <p>When more than one coroutine is blocked in <a class="reference internal" href="#asyncio.Lock.acquire" title="asyncio.Lock.acquire"><code class="xref py py-meth docutils literal notranslate"><span class="pre">acquire()</span></code></a>

  260. waiting for the lock to be unlocked, only one coroutine

  261. eventually proceeds.</p>

  262. <p>Acquiring a lock is <em>fair</em>: the coroutine that proceeds will be

  263. the first coroutine that started waiting on the lock.</p>

  264. </dd></dl>

  265. 

  266. <dl class="py method">

  267. <dt class="sig sig-object py" id="asyncio.Lock.release">

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

  269. <dd><p>Release the lock.</p>

  270. <p>When the lock is <em>locked</em>, reset it to <em>unlocked</em> and return.</p>

  271. <p>If the lock is <em>unlocked</em>, 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>

  272. </dd></dl>

  273. 

  274. <dl class="py method">

  275. <dt class="sig sig-object py" id="asyncio.Lock.locked">

  276. <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="#asyncio.Lock.locked" title="Link to this definition">¶</a></dt>

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

  278. </dd></dl>

  279. 

  280. </dd></dl>

  281. 

  282. </section>

  283. <section id="event">

  284. <h2>Event<a class="headerlink" href="#event" title="Link to this heading">¶</a></h2>

  285. <dl class="py class">

  286. <dt class="sig sig-object py" id="asyncio.Event">

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

  288. <dd><p>An event object.  Not thread-safe.</p>

  289. <p>An asyncio event can be used to notify multiple asyncio tasks

  290. that some event has happened.</p>

  291. <p>An Event object manages an internal flag that can be set to <em>true</em>

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

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

  294. flag is set to <em>true</em>.  The flag is set to <em>false</em> initially.</p>

  295. <div class="versionchanged">

  296. <p><span class="versionmodified changed">Changed in version 3.10: </span>Removed the <em>loop</em> parameter.</p>

  297. </div>

  298. <p id="asyncio-example-sync-event">Example:</p>

  299. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span> <span class="nf">waiter</span><span class="p">(</span><span class="n">event</span><span class="p">):</span>

  300.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;waiting for it ...&#39;</span><span class="p">)</span>

  301.     <span class="k">await</span> <span class="n">event</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>

  302.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;... got it!&#39;</span><span class="p">)</span>

  303. 

  304. <span class="k">async</span> <span class="k">def</span> <span class="nf">main</span><span class="p">():</span>

  305.     <span class="c1"># Create an Event object.</span>

  306.     <span class="n">event</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>

  307. 

  308.     <span class="c1"># Spawn a Task to wait until &#39;event&#39; is set.</span>

  309.     <span class="n">waiter_task</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">waiter</span><span class="p">(</span><span class="n">event</span><span class="p">))</span>

  310. 

  311.     <span class="c1"># Sleep for 1 second and set the event.</span>

  312.     <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>

  313.     <span class="n">event</span><span class="o">.</span><span class="n">set</span><span class="p">()</span>

  314. 

  315.     <span class="c1"># Wait until the waiter task is finished.</span>

  316.     <span class="k">await</span> <span class="n">waiter_task</span>

  317. 

  318. <span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>

  319. </pre></div>

  320. </div>

  321. <dl class="py method">

  322. <dt class="sig sig-object py" id="asyncio.Event.wait">

  323. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">wait</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Event.wait" title="Link to this definition">¶</a></dt>

  324. <dd><p>Wait until the event is set.</p>

  325. <p>If the event is set, return <code class="docutils literal notranslate"><span class="pre">True</span></code> immediately.

  326. Otherwise block until another task calls <a class="reference internal" href="#asyncio.Event.set" title="asyncio.Event.set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set()</span></code></a>.</p>

  327. </dd></dl>

  328. 

  329. <dl class="py method">

  330. <dt class="sig sig-object py" id="asyncio.Event.set">

  331. <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="#asyncio.Event.set" title="Link to this definition">¶</a></dt>

  332. <dd><p>Set the event.</p>

  333. <p>All tasks waiting for event to be set will be immediately

  334. awakened.</p>

  335. </dd></dl>

  336. 

  337. <dl class="py method">

  338. <dt class="sig sig-object py" id="asyncio.Event.clear">

  339. <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="#asyncio.Event.clear" title="Link to this definition">¶</a></dt>

  340. <dd><p>Clear (unset) the event.</p>

  341. <p>Tasks awaiting on <a class="reference internal" href="#asyncio.Event.wait" title="asyncio.Event.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> will now block until the

  342. <a class="reference internal" href="#asyncio.Event.set" title="asyncio.Event.set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set()</span></code></a> method is called again.</p>

  343. </dd></dl>

  344. 

  345. <dl class="py method">

  346. <dt class="sig sig-object py" id="asyncio.Event.is_set">

  347. <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="#asyncio.Event.is_set" title="Link to this definition">¶</a></dt>

  348. <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the event is set.</p>

  349. </dd></dl>

  350. 

  351. </dd></dl>

  352. 

  353. </section>

  354. <section id="condition">

  355. <h2>Condition<a class="headerlink" href="#condition" title="Link to this heading">¶</a></h2>

  356. <dl class="py class">

  357. <dt class="sig sig-object py" id="asyncio.Condition">

  358. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</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="#asyncio.Condition" title="Link to this definition">¶</a></dt>

  359. <dd><p>A Condition object.  Not thread-safe.</p>

  360. <p>An asyncio condition primitive can be used by a task to wait for

  361. some event to happen and then get exclusive access to a shared

  362. resource.</p>

  363. <p>In essence, a Condition object combines the functionality

  364. of an <a class="reference internal" href="#asyncio.Event" title="asyncio.Event"><code class="xref py py-class docutils literal notranslate"><span class="pre">Event</span></code></a> and a <a class="reference internal" href="#asyncio.Lock" title="asyncio.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a>.  It is possible to have

  365. multiple Condition objects share one Lock, which allows coordinating

  366. exclusive access to a shared resource between different tasks

  367. interested in particular states of that shared resource.</p>

  368. <p>The optional <em>lock</em> argument must be a <a class="reference internal" href="#asyncio.Lock" title="asyncio.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> object or

  369. <code class="docutils literal notranslate"><span class="pre">None</span></code>.  In the latter case a new Lock object is created

  370. automatically.</p>

  371. <div class="versionchanged">

  372. <p><span class="versionmodified changed">Changed in version 3.10: </span>Removed the <em>loop</em> parameter.</p>

  373. </div>

  374. <p>The preferred way to use a Condition is an <a class="reference internal" href="../reference/compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a>

  375. statement:</p>

  376. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">cond</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Condition</span><span class="p">()</span>

  377. 

  378. <span class="c1"># ... later</span>

  379. <span class="k">async</span> <span class="k">with</span> <span class="n">cond</span><span class="p">:</span>

  380.     <span class="k">await</span> <span class="n">cond</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>

  381. </pre></div>

  382. </div>

  383. <p>which is equivalent to:</p>

  384. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">cond</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Condition</span><span class="p">()</span>

  385. 

  386. <span class="c1"># ... later</span>

  387. <span class="k">await</span> <span class="n">cond</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>

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

  389.     <span class="k">await</span> <span class="n">cond</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>

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

  391.     <span class="n">cond</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>

  392. </pre></div>

  393. </div>

  394. <dl class="py method">

  395. <dt class="sig sig-object py" id="asyncio.Condition.acquire">

  396. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">acquire</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.acquire" title="Link to this definition">¶</a></dt>

  397. <dd><p>Acquire the underlying lock.</p>

  398. <p>This method waits until the underlying lock is <em>unlocked</em>,

  399. sets it to <em>locked</em> and returns <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  400. </dd></dl>

  401. 

  402. <dl class="py method">

  403. <dt class="sig sig-object py" id="asyncio.Condition.notify">

  404. <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="#asyncio.Condition.notify" title="Link to this definition">¶</a></dt>

  405. <dd><p>Wake up at most <em>n</em> tasks (1 by default) waiting on this

  406. condition.  The method is no-op if no tasks are waiting.</p>

  407. <p>The lock must be acquired before this method is called and

  408. released shortly after.  If called with an <em>unlocked</em> lock

  409. 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> error is raised.</p>

  410. </dd></dl>

  411. 

  412. <dl class="py method">

  413. <dt class="sig sig-object py" id="asyncio.Condition.locked">

  414. <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="#asyncio.Condition.locked" title="Link to this definition">¶</a></dt>

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

  416. </dd></dl>

  417. 

  418. <dl class="py method">

  419. <dt class="sig sig-object py" id="asyncio.Condition.notify_all">

  420. <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="#asyncio.Condition.notify_all" title="Link to this definition">¶</a></dt>

  421. <dd><p>Wake up all tasks waiting on this condition.</p>

  422. <p>This method acts like <a class="reference internal" href="#asyncio.Condition.notify" title="asyncio.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a>, but wakes up all waiting

  423. tasks.</p>

  424. <p>The lock must be acquired before this method is called and

  425. released shortly after.  If called with an <em>unlocked</em> lock

  426. 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> error is raised.</p>

  427. </dd></dl>

  428. 

  429. <dl class="py method">

  430. <dt class="sig sig-object py" id="asyncio.Condition.release">

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

  432. <dd><p>Release the underlying lock.</p>

  433. <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

  434. raised.</p>

  435. </dd></dl>

  436. 

  437. <dl class="py method">

  438. <dt class="sig sig-object py" id="asyncio.Condition.wait">

  439. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">wait</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.wait" title="Link to this definition">¶</a></dt>

  440. <dd><p>Wait until notified.</p>

  441. <p>If the calling task has not acquired the lock when this method is

  442. 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 raised.</p>

  443. <p>This method releases the underlying lock, and then blocks until

  444. it is awakened by a <a class="reference internal" href="#asyncio.Condition.notify" title="asyncio.Condition.notify"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify()</span></code></a> or <a class="reference internal" href="#asyncio.Condition.notify_all" title="asyncio.Condition.notify_all"><code class="xref py py-meth docutils literal notranslate"><span class="pre">notify_all()</span></code></a> call.

  445. Once awakened, the Condition re-acquires its lock and this method

  446. returns <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  447. </dd></dl>

  448. 

  449. <dl class="py method">

  450. <dt class="sig sig-object py" id="asyncio.Condition.wait_for">

  451. <em class="property"><span class="pre">coroutine</span> </em><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><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.wait_for" title="Link to this definition">¶</a></dt>

  452. <dd><p>Wait until a predicate becomes <em>true</em>.</p>

  453. <p>The predicate must be a callable which result will be

  454. interpreted as a boolean value.  The final value is the

  455. return value.</p>

  456. </dd></dl>

  457. 

  458. </dd></dl>

  459. 

  460. </section>

  461. <section id="semaphore">

  462. <h2>Semaphore<a class="headerlink" href="#semaphore" title="Link to this heading">¶</a></h2>

  463. <dl class="py class">

  464. <dt class="sig sig-object py" id="asyncio.Semaphore">

  465. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</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="#asyncio.Semaphore" title="Link to this definition">¶</a></dt>

  466. <dd><p>A Semaphore object.  Not thread-safe.</p>

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

  468. <a class="reference internal" href="#asyncio.Semaphore.acquire" title="asyncio.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="#asyncio.Semaphore.release" title="asyncio.Semaphore.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> call.

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

  470. that it is zero, it blocks, waiting until some task calls

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

  472. <p>The optional <em>value</em> argument gives the initial value for the

  473. internal counter (<code class="docutils literal notranslate"><span class="pre">1</span></code> by default). If the given value is

  474. less than <code class="docutils literal notranslate"><span class="pre">0</span></code> a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised.</p>

  475. <div class="versionchanged">

  476. <p><span class="versionmodified changed">Changed in version 3.10: </span>Removed the <em>loop</em> parameter.</p>

  477. </div>

  478. <p>The preferred way to use a Semaphore is an <a class="reference internal" href="../reference/compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a>

  479. statement:</p>

  480. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">sem</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Semaphore</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>

  481. 

  482. <span class="c1"># ... later</span>

  483. <span class="k">async</span> <span class="k">with</span> <span class="n">sem</span><span class="p">:</span>

  484.     <span class="c1"># work with shared resource</span>

  485. </pre></div>

  486. </div>

  487. <p>which is equivalent to:</p>

  488. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">sem</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Semaphore</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>

  489. 

  490. <span class="c1"># ... later</span>

  491. <span class="k">await</span> <span class="n">sem</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>

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

  493.     <span class="c1"># work with shared resource</span>

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

  495.     <span class="n">sem</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>

  496. </pre></div>

  497. </div>

  498. <dl class="py method">

  499. <dt class="sig sig-object py" id="asyncio.Semaphore.acquire">

  500. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">acquire</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Semaphore.acquire" title="Link to this definition">¶</a></dt>

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

  502. <p>If the internal counter is greater than zero, decrement

  503. it by one and return <code class="docutils literal notranslate"><span class="pre">True</span></code> immediately.  If it is zero, wait

  504. until a <a class="reference internal" href="#asyncio.Semaphore.release" title="asyncio.Semaphore.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> is called and return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  505. </dd></dl>

  506. 

  507. <dl class="py method">

  508. <dt class="sig sig-object py" id="asyncio.Semaphore.locked">

  509. <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="#asyncio.Semaphore.locked" title="Link to this definition">¶</a></dt>

  510. <dd><p>Returns <code class="docutils literal notranslate"><span class="pre">True</span></code> if semaphore can not be acquired immediately.</p>

  511. </dd></dl>

  512. 

  513. <dl class="py method">

  514. <dt class="sig sig-object py" id="asyncio.Semaphore.release">

  515. <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="#asyncio.Semaphore.release" title="Link to this definition">¶</a></dt>

  516. <dd><p>Release a semaphore, incrementing the internal counter by one.

  517. Can wake up a task waiting to acquire the semaphore.</p>

  518. <p>Unlike <a class="reference internal" href="#asyncio.BoundedSemaphore" title="asyncio.BoundedSemaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">BoundedSemaphore</span></code></a>, <a class="reference internal" href="#asyncio.Semaphore" title="asyncio.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">Semaphore</span></code></a> allows

  519. making more <code class="docutils literal notranslate"><span class="pre">release()</span></code> calls than <code class="docutils literal notranslate"><span class="pre">acquire()</span></code> calls.</p>

  520. </dd></dl>

  521. 

  522. </dd></dl>

  523. 

  524. </section>

  525. <section id="boundedsemaphore">

  526. <h2>BoundedSemaphore<a class="headerlink" href="#boundedsemaphore" title="Link to this heading">¶</a></h2>

  527. <dl class="py class">

  528. <dt class="sig sig-object py" id="asyncio.BoundedSemaphore">

  529. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</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="#asyncio.BoundedSemaphore" title="Link to this definition">¶</a></dt>

  530. <dd><p>A bounded semaphore object.  Not thread-safe.</p>

  531. <p>Bounded Semaphore is a version of <a class="reference internal" href="#asyncio.Semaphore" title="asyncio.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">Semaphore</span></code></a> that raises

  532. 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> in <a class="reference internal" href="#asyncio.Semaphore.release" title="asyncio.Semaphore.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a> if it

  533. increases the internal counter above the initial <em>value</em>.</p>

  534. <div class="versionchanged">

  535. <p><span class="versionmodified changed">Changed in version 3.10: </span>Removed the <em>loop</em> parameter.</p>

  536. </div>

  537. </dd></dl>

  538. 

  539. </section>

  540. <section id="barrier">

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

  542. <dl class="py class">

  543. <dt class="sig sig-object py" id="asyncio.Barrier">

  544. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">Barrier</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">parties</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Barrier" title="Link to this definition">¶</a></dt>

  545. <dd><p>A barrier object.  Not thread-safe.</p>

  546. <p>A barrier is a simple synchronization primitive that allows to block until

  547. <em>parties</em> number of tasks are waiting on it.

  548. Tasks can wait on the <a class="reference internal" href="#asyncio.Barrier.wait" title="asyncio.Barrier.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> method and would be blocked until

  549. the specified number of tasks end up waiting on <a class="reference internal" href="#asyncio.Barrier.wait" title="asyncio.Barrier.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a>.

  550. At that point all of the waiting tasks would unblock simultaneously.</p>

  551. <p><a class="reference internal" href="../reference/compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a> can be used as an alternative to awaiting on

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

  553. <p>The barrier can be reused any number of times.</p>

  554. <p id="asyncio-example-barrier">Example:</p>

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

  556.    <span class="c1"># barrier with 3 parties</span>

  557.    <span class="n">b</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Barrier</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>

  558. 

  559.    <span class="c1"># create 2 new waiting tasks</span>

  560.    <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">b</span><span class="o">.</span><span class="n">wait</span><span class="p">())</span>

  561.    <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">b</span><span class="o">.</span><span class="n">wait</span><span class="p">())</span>

  562. 

  563.    <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>

  564.    <span class="nb">print</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>

  565. 

  566.    <span class="c1"># The third .wait() call passes the barrier</span>

  567.    <span class="k">await</span> <span class="n">b</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>

  568.    <span class="nb">print</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>

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

  570. 

  571.    <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>

  572.    <span class="nb">print</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>

  573. 

  574. <span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">example_barrier</span><span class="p">())</span>

  575. </pre></div>

  576. </div>

  577. <p>Result of this example is:</p>

  578. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="o">&lt;</span><span class="n">asyncio</span><span class="o">.</span><span class="n">locks</span><span class="o">.</span><span class="n">Barrier</span> <span class="nb">object</span> <span class="n">at</span> <span class="mi">0</span><span class="n">x</span><span class="o">...</span> <span class="p">[</span><span class="n">filling</span><span class="p">,</span> <span class="n">waiters</span><span class="p">:</span><span class="mi">2</span><span class="o">/</span><span class="mi">3</span><span class="p">]</span><span class="o">&gt;</span>

  579. <span class="o">&lt;</span><span class="n">asyncio</span><span class="o">.</span><span class="n">locks</span><span class="o">.</span><span class="n">Barrier</span> <span class="nb">object</span> <span class="n">at</span> <span class="mi">0</span><span class="n">x</span><span class="o">...</span> <span class="p">[</span><span class="n">draining</span><span class="p">,</span> <span class="n">waiters</span><span class="p">:</span><span class="mi">0</span><span class="o">/</span><span class="mi">3</span><span class="p">]</span><span class="o">&gt;</span>

  580. <span class="n">barrier</span> <span class="n">passed</span>

  581. <span class="o">&lt;</span><span class="n">asyncio</span><span class="o">.</span><span class="n">locks</span><span class="o">.</span><span class="n">Barrier</span> <span class="nb">object</span> <span class="n">at</span> <span class="mi">0</span><span class="n">x</span><span class="o">...</span> <span class="p">[</span><span class="n">filling</span><span class="p">,</span> <span class="n">waiters</span><span class="p">:</span><span class="mi">0</span><span class="o">/</span><span class="mi">3</span><span class="p">]</span><span class="o">&gt;</span>

  582. </pre></div>

  583. </div>

  584. <div class="versionadded">

  585. <p><span class="versionmodified added">New in version 3.11.</span></p>

  586. </div>

  587. <dl class="py method">

  588. <dt class="sig sig-object py" id="asyncio.Barrier.wait">

  589. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">wait</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Barrier.wait" title="Link to this definition">¶</a></dt>

  590. <dd><p>Pass the barrier. When all the tasks party to the barrier have called

  591. this function, they are all unblocked simultaneously.</p>

  592. <p>When a waiting or blocked task in the barrier is cancelled,

  593. this task exits the barrier which stays in the same state.

  594. If the state of the barrier is “filling”, the number of waiting task

  595. decreases by 1.</p>

  596. <p>The return value is an integer in the range of 0 to <code class="docutils literal notranslate"><span class="pre">parties-1</span></code>, different

  597. for each task. This can be used to select a task to do some special

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

  599. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="o">...</span>

  600. <span class="k">async</span> <span class="k">with</span> <span class="n">barrier</span> <span class="k">as</span> <span class="n">position</span><span class="p">:</span>

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

  602.       <span class="c1"># Only one task prints this</span>

  603.       <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;End of *draining phase*&#39;</span><span class="p">)</span>

  604. </pre></div>

  605. </div>

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

  607. barrier is broken or reset while a task is waiting.

  608. It could raise a <a class="reference internal" href="asyncio-exceptions.html#asyncio.CancelledError" title="asyncio.CancelledError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">CancelledError</span></code></a> if a task is cancelled.</p>

  609. </dd></dl>

  610. 

  611. <dl class="py method">

  612. <dt class="sig sig-object py" id="asyncio.Barrier.reset">

  613. <em class="property"><span class="pre">coroutine</span> </em><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="#asyncio.Barrier.reset" title="Link to this definition">¶</a></dt>

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

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

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

  617. </dd></dl>

  618. 

  619. <dl class="py method">

  620. <dt class="sig sig-object py" id="asyncio.Barrier.abort">

  621. <em class="property"><span class="pre">coroutine</span> </em><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="#asyncio.Barrier.abort" title="Link to this definition">¶</a></dt>

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

  623. calls to <a class="reference internal" href="asyncio-task.html#asyncio.wait" title="asyncio.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="#asyncio.BrokenBarrierError" title="asyncio.BrokenBarrierError"><code class="xref py py-class docutils literal notranslate"><span class="pre">BrokenBarrierError</span></code></a>.

  624. Use this for example if one of the tasks needs to abort, to avoid infinite

  625. waiting tasks.</p>

  626. </dd></dl>

  627. 

  628. <dl class="py attribute">

  629. <dt class="sig sig-object py" id="asyncio.Barrier.parties">

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

  631. <dd><p>The number of tasks required to pass the barrier.</p>

  632. </dd></dl>

  633. 

  634. <dl class="py attribute">

  635. <dt class="sig sig-object py" id="asyncio.Barrier.n_waiting">

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

  637. <dd><p>The number of tasks currently waiting in the barrier while filling.</p>

  638. </dd></dl>

  639. 

  640. <dl class="py attribute">

  641. <dt class="sig sig-object py" id="asyncio.Barrier.broken">

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

  643. <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>

  644. </dd></dl>

  645. 

  646. </dd></dl>

  647. 

  648. <dl class="py exception">

  649. <dt class="sig sig-object py" id="asyncio.BrokenBarrierError">

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

  651. <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

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

  653. </dd></dl>

  654. 

  655. <hr class="docutils" />

  656. <div class="versionchanged">

  657. <p><span class="versionmodified changed">Changed in version 3.9: </span>Acquiring a lock using <code class="docutils literal notranslate"><span class="pre">await</span> <span class="pre">lock</span></code> or <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span> <span class="pre">lock</span></code> and/or

  658. <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 (<code class="docutils literal notranslate"><span class="pre">with</span> <span class="pre">await</span> <span class="pre">lock</span></code>, <code class="docutils literal notranslate"><span class="pre">with</span> <span class="pre">(yield</span> <span class="pre">from</span>

  659. <span class="pre">lock)</span></code>) was removed.  Use <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span> <span class="pre">lock</span></code> instead.</p>

  660. </div>

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  673.     <ul>

  674. <li><a class="reference internal" href="#">Synchronization Primitives</a><ul>

  675. <li><a class="reference internal" href="#lock">Lock</a></li>

  676. <li><a class="reference internal" href="#event">Event</a></li>

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