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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>18.5.7. Synchronization primitives — Python 3.6.7 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.6.7 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="18.5.8. Queues" href="asyncio-queue.html" /> <link rel="prev" title="18.5.6. Subprocess" href="asyncio-subprocess.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/library/asyncio-sync.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="asyncio-queue.html" title="18.5.8. Queues" accesskey="N">next</a> |</li> <li class="right" > <a href="asyncio-subprocess.html" title="18.5.6. Subprocess" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="ipc.html" >18. Interprocess Communication and Networking</a> »</li> <li class="nav-item nav-item-3"><a href="asyncio.html" accesskey="U">18.5. <code class="docutils literal notranslate"><span class="pre">asyncio</span></code> — Asynchronous I/O, event loop, coroutines and tasks</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="synchronization-primitives"> <span id="asyncio-sync"></span><h1>18.5.7. Synchronization primitives<a class="headerlink" href="#synchronization-primitives" title="Permalink to this headline">¶</a></h1> <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/asyncio/locks.py">Lib/asyncio/locks.py</a></p> <p>Locks:</p> <ul class="simple"> <li><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></li> <li><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></li> <li><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></li> </ul> <p>Semaphores:</p> <ul class="simple"> <li><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></li> <li><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></li> </ul> <p>asyncio lock API was designed to be close to classes 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 (<a class="reference internal" href="threading.html#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.html#threading.Event" title="threading.Event"><code class="xref py py-class docutils literal notranslate"><span class="pre">Event</span></code></a>, <a class="reference internal" href="threading.html#threading.Condition" title="threading.Condition"><code class="xref py py-class docutils literal notranslate"><span class="pre">Condition</span></code></a>, <a class="reference internal" href="threading.html#threading.Semaphore" title="threading.Semaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">Semaphore</span></code></a>, <a class="reference internal" href="threading.html#threading.BoundedSemaphore" title="threading.BoundedSemaphore"><code class="xref py py-class docutils literal notranslate"><span class="pre">BoundedSemaphore</span></code></a>), but it has no <em>timeout</em> parameter. 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 can be used to cancel a task after a timeout.</p> <div class="section" id="locks"> <h2>18.5.7.1. Locks<a class="headerlink" href="#locks" title="Permalink to this headline">¶</a></h2> <div class="section" id="lock"> <h3>18.5.7.1.1. Lock<a class="headerlink" href="#lock" title="Permalink to this headline">¶</a></h3> <dl class="class"> <dt id="asyncio.Lock"> <em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">Lock</code><span class="sig-paren">(</span><em>*</em>, <em>loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Lock" title="Permalink to this definition">¶</a></dt> <dd><p>Primitive lock objects.</p> <p>A primitive lock is a synchronization primitive that is not owned by a particular coroutine when locked. A primitive lock is in one of two states, ‘locked’ or ‘unlocked’.</p> <p>It is created in the unlocked state. It has two basic methods, <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> and <a class="reference internal" href="#asyncio.Lock.release" title="asyncio.Lock.release"><code class="xref py py-meth docutils literal notranslate"><span class="pre">release()</span></code></a>. When the state is unlocked, acquire() changes the state to locked and returns immediately. When the state is locked, acquire() blocks until a call to release() in another coroutine changes it to unlocked, then the acquire() call resets it to locked and returns. The release() method should only be called in the locked state; it changes the state to unlocked and returns immediately. If an attempt is made to release 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> will be raised.</p> <p>When more than one coroutine is blocked in acquire() waiting for the state to turn to unlocked, only one coroutine proceeds when a release() call resets the state to unlocked; first coroutine which is blocked in acquire() is being processed.</p> <p><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> is a coroutine and should be called with <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code>.</p> <p>Locks also support the context management protocol. <code class="docutils literal notranslate"><span class="pre">(yield</span> <span class="pre">from</span> <span class="pre">lock)</span></code> should be used as the context manager expression.</p> <p>This class is <a class="reference internal" href="asyncio-dev.html#asyncio-multithreading"><span class="std std-ref">not thread safe</span></a>.</p> <p>Usage:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">lock</span> <span class="o">=</span> <span class="n">Lock</span><span class="p">()</span> <span class="o">...</span> <span class="k">yield from</span> <span class="n">lock</span> <span class="k">try</span><span class="p">:</span> <span class="o">...</span> <span class="k">finally</span><span class="p">:</span> <span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span> </pre></div> </div> <p>Context manager usage:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">lock</span> <span class="o">=</span> <span class="n">Lock</span><span class="p">()</span> <span class="o">...</span> <span class="k">with</span> <span class="p">(</span><span class="k">yield from</span> <span class="n">lock</span><span class="p">):</span> <span class="o">...</span> </pre></div> </div> <p>Lock objects can be tested for locking state:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="ow">not</span> <span class="n">lock</span><span class="o">.</span><span class="n">locked</span><span class="p">():</span> <span class="k">yield from</span> <span class="n">lock</span> <span class="k">else</span><span class="p">:</span> <span class="c1"># lock is acquired</span> <span class="o">...</span> </pre></div> </div> <dl class="method"> <dt id="asyncio.Lock.locked"> <code class="descname">locked</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Lock.locked" title="Permalink to this definition">¶</a></dt> <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the lock is acquired.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Lock.acquire"> <em class="property">coroutine </em><code class="descname">acquire</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Lock.acquire" title="Permalink to this definition">¶</a></dt> <dd><p>Acquire a lock.</p> <p>This method blocks until the lock is unlocked, then sets it to locked and returns <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p> <p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span class="std std-ref">coroutine</span></a>.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Lock.release"> <code class="descname">release</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Lock.release" title="Permalink to this definition">¶</a></dt> <dd><p>Release a lock.</p> <p>When the lock is locked, reset it to unlocked, and return. If any other coroutines are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed.</p> <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> <p>There is no return value.</p> </dd></dl> </dd></dl> </div> <div class="section" id="event"> <h3>18.5.7.1.2. Event<a class="headerlink" href="#event" title="Permalink to this headline">¶</a></h3> <dl class="class"> <dt id="asyncio.Event"> <em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">Event</code><span class="sig-paren">(</span><em>*</em>, <em>loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Event" title="Permalink to this definition">¶</a></dt> <dd><p>An Event implementation, asynchronous equivalent to <a class="reference internal" href="threading.html#threading.Event" title="threading.Event"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Event</span></code></a>.</p> <p>Class implementing event objects. An event manages a flag that can be set to true with the <a class="reference internal" href="stdtypes.html#set" title="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="#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-task.html#asyncio.wait" title="asyncio.wait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code></a> method blocks until the flag is true. The flag is initially false.</p> <p>This class is <a class="reference internal" href="asyncio-dev.html#asyncio-multithreading"><span class="std std-ref">not thread safe</span></a>.</p> <dl class="method"> <dt id="asyncio.Event.clear"> <code class="descname">clear</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Event.clear" title="Permalink to this definition">¶</a></dt> <dd><p>Reset the internal flag to false. Subsequently, coroutines calling <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> will block until <a class="reference internal" href="stdtypes.html#set" title="set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set()</span></code></a> is called to set the internal flag to true again.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Event.is_set"> <code class="descname">is_set</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Event.is_set" title="Permalink to this definition">¶</a></dt> <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if and only if the internal flag is true.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Event.set"> <code class="descname">set</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Event.set" title="Permalink to this definition">¶</a></dt> <dd><p>Set the internal flag to true. All coroutines waiting for it to become true are awakened. Coroutine that call <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> once the flag is true will not block at all.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Event.wait"> <em class="property">coroutine </em><code class="descname">wait</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Event.wait" title="Permalink to this definition">¶</a></dt> <dd><p>Block until the internal flag is true.</p> <p>If the internal flag is true on entry, return <code class="docutils literal notranslate"><span class="pre">True</span></code> immediately. Otherwise, block until another coroutine calls <a class="reference internal" href="stdtypes.html#set" title="set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set()</span></code></a> to set the flag to true, then return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p> <p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span class="std std-ref">coroutine</span></a>.</p> </dd></dl> </dd></dl> </div> <div class="section" id="condition"> <h3>18.5.7.1.3. Condition<a class="headerlink" href="#condition" title="Permalink to this headline">¶</a></h3> <dl class="class"> <dt id="asyncio.Condition"> <em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">Condition</code><span class="sig-paren">(</span><em>lock=None</em>, <em>*</em>, <em>loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition" title="Permalink to this definition">¶</a></dt> <dd><p>A Condition implementation, asynchronous equivalent to <a class="reference internal" href="threading.html#threading.Condition" title="threading.Condition"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Condition</span></code></a>.</p> <p>This class implements condition variable objects. A condition variable allows one or more coroutines to wait until they are notified by another coroutine.</p> <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="#asyncio.Lock" title="asyncio.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">Lock</span></code></a> object, and it is used as the underlying lock. Otherwise, a new <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 is created and used as the underlying lock.</p> <p>This class is <a class="reference internal" href="asyncio-dev.html#asyncio-multithreading"><span class="std std-ref">not thread safe</span></a>.</p> <dl class="method"> <dt id="asyncio.Condition.acquire"> <em class="property">coroutine </em><code class="descname">acquire</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.acquire" title="Permalink to this definition">¶</a></dt> <dd><p>Acquire the underlying lock.</p> <p>This method blocks until the lock is unlocked, then sets it to locked and returns <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p> <p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span class="std std-ref">coroutine</span></a>.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Condition.notify"> <code class="descname">notify</code><span class="sig-paren">(</span><em>n=1</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.notify" title="Permalink to this definition">¶</a></dt> <dd><p>By default, wake up one coroutine waiting on this condition, if any. If the calling coroutine has 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 raised.</p> <p>This method wakes up at most <em>n</em> of the coroutines waiting for the condition variable; it is a no-op if no coroutines are waiting.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">An awakened coroutine does not actually return from its <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> call until it can reacquire the lock. Since <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> does not release the lock, its caller should.</p> </div> </dd></dl> <dl class="method"> <dt id="asyncio.Condition.locked"> <code class="descname">locked</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.locked" title="Permalink to this definition">¶</a></dt> <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the underlying lock is acquired.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Condition.notify_all"> <code class="descname">notify_all</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.notify_all" title="Permalink to this definition">¶</a></dt> <dd><p>Wake up all coroutines waiting on this condition. 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 coroutines instead of one. If the calling coroutine has 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 raised.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Condition.release"> <code class="descname">release</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.release" title="Permalink to this definition">¶</a></dt> <dd><p>Release the underlying lock.</p> <p>When the lock is locked, reset it to unlocked, and return. If any other coroutines are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed.</p> <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> <p>There is no return value.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Condition.wait"> <em class="property">coroutine </em><code class="descname">wait</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.wait" title="Permalink to this definition">¶</a></dt> <dd><p>Wait until notified.</p> <p>If the calling coroutine has 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 raised.</p> <p>This method releases the underlying lock, and then blocks until 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 for the same condition variable in another coroutine. Once awakened, it re-acquires the lock and returns <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p> <p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span class="std std-ref">coroutine</span></a>.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Condition.wait_for"> <em class="property">coroutine </em><code class="descname">wait_for</code><span class="sig-paren">(</span><em>predicate</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Condition.wait_for" title="Permalink to this definition">¶</a></dt> <dd><p>Wait until a predicate becomes true.</p> <p>The predicate should be a callable which result will be interpreted as a boolean value. The final predicate value is the return value.</p> <p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span class="std std-ref">coroutine</span></a>.</p> </dd></dl> </dd></dl> </div> </div> <div class="section" id="semaphores"> <h2>18.5.7.2. Semaphores<a class="headerlink" href="#semaphores" title="Permalink to this headline">¶</a></h2> <div class="section" id="semaphore"> <h3>18.5.7.2.1. Semaphore<a class="headerlink" href="#semaphore" title="Permalink to this headline">¶</a></h3> <dl class="class"> <dt id="asyncio.Semaphore"> <em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">Semaphore</code><span class="sig-paren">(</span><em>value=1</em>, <em>*</em>, <em>loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Semaphore" title="Permalink to this definition">¶</a></dt> <dd><p>A Semaphore implementation.</p> <p>A semaphore manages an internal counter which is decremented by each <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. 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 that it is zero, it blocks, waiting until some other coroutine calls <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> <p>Semaphores also support the context management protocol.</p> <p>The optional argument gives the initial value for the internal counter; it defaults to <code class="docutils literal notranslate"><span class="pre">1</span></code>. If the value given is less than <code class="docutils literal notranslate"><span class="pre">0</span></code>, <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised.</p> <p>This class is <a class="reference internal" href="asyncio-dev.html#asyncio-multithreading"><span class="std std-ref">not thread safe</span></a>.</p> <dl class="method"> <dt id="asyncio.Semaphore.acquire"> <em class="property">coroutine </em><code class="descname">acquire</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Semaphore.acquire" title="Permalink to this definition">¶</a></dt> <dd><p>Acquire a semaphore.</p> <p>If the internal counter is larger than zero on entry, decrement it by one and return <code class="docutils literal notranslate"><span class="pre">True</span></code> immediately. If it is zero on entry, block, waiting until some other coroutine has called <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> to make it larger than <code class="docutils literal notranslate"><span class="pre">0</span></code>, and then return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p> <p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span class="std std-ref">coroutine</span></a>.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Semaphore.locked"> <code class="descname">locked</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Semaphore.locked" title="Permalink to this definition">¶</a></dt> <dd><p>Returns <code class="docutils literal notranslate"><span class="pre">True</span></code> if semaphore can not be acquired immediately.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Semaphore.release"> <code class="descname">release</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Semaphore.release" title="Permalink to this definition">¶</a></dt> <dd><p>Release a semaphore, incrementing the internal counter by one. When it was zero on entry and another coroutine is waiting for it to become larger than zero again, wake up that coroutine.</p> </dd></dl> </dd></dl> </div> <div class="section" id="boundedsemaphore"> <h3>18.5.7.2.2. BoundedSemaphore<a class="headerlink" href="#boundedsemaphore" title="Permalink to this headline">¶</a></h3> <dl class="class"> <dt id="asyncio.BoundedSemaphore"> <em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">BoundedSemaphore</code><span class="sig-paren">(</span><em>value=1</em>, <em>*</em>, <em>loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.BoundedSemaphore" title="Permalink to this definition">¶</a></dt> <dd><p>A bounded semaphore implementation. Inherit from <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> <p>This raises <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 would increase the value above the initial value.</p> </dd></dl> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">18.5.7. Synchronization primitives</a><ul> <li><a class="reference internal" href="#locks">18.5.7.1. Locks</a><ul> <li><a class="reference internal" href="#lock">18.5.7.1.1. Lock</a></li> <li><a class="reference internal" href="#event">18.5.7.1.2. Event</a></li> <li><a class="reference internal" href="#condition">18.5.7.1.3. Condition</a></li> </ul> </li> <li><a class="reference internal" href="#semaphores">18.5.7.2. Semaphores</a><ul> <li><a class="reference internal" href="#semaphore">18.5.7.2.1. Semaphore</a></li> <li><a class="reference internal" href="#boundedsemaphore">18.5.7.2.2. BoundedSemaphore</a></li> </ul> </li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="asyncio-subprocess.html" title="previous chapter">18.5.6. Subprocess</a></p> <h4>Next topic</h4> <p class="topless"><a href="asyncio-queue.html" title="next chapter">18.5.8. 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