<!DOCTYPE html> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta charset="utf-8" /> <title>Policies — Python 3.7.10 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/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.7.10 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="Platform Support" href="asyncio-platforms.html" /> <link rel="prev" title="Transports and Protocols" href="asyncio-protocol.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/library/asyncio-policy.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> <style> @media only screen { table.full-width-table { width: 100%; } } </style> </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-platforms.html" title="Platform Support" accesskey="N">next</a> |</li> <li class="right" > <a href="asyncio-protocol.html" title="Transports and Protocols" 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">3.7.10 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" >Networking and Interprocess Communication</a> »</li> <li class="nav-item nav-item-3"><a href="asyncio.html" accesskey="U"><code class="xref py py-mod docutils literal notranslate"><span class="pre">asyncio</span></code> — Asynchronous I/O</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="policies"> <span id="asyncio-policies"></span><h1>Policies<a class="headerlink" href="#policies" title="Permalink to this headline">¶</a></h1> <p>An event loop policy is a global per-process object that controls the management of the event loop. Each event loop has a default policy, which can be changed and customized using the policy API.</p> <p>A policy defines the notion of <em>context</em> and manages a separate event loop per context. The default policy defines <em>context</em> to be the current thread.</p> <p>By using a custom event loop policy, the behavior of <a class="reference internal" href="asyncio-eventloop.html#asyncio.get_event_loop" title="asyncio.get_event_loop"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_event_loop()</span></code></a>, <a class="reference internal" href="asyncio-eventloop.html#asyncio.set_event_loop" title="asyncio.set_event_loop"><code class="xref py py-func docutils literal notranslate"><span class="pre">set_event_loop()</span></code></a>, and <a class="reference internal" href="asyncio-eventloop.html#asyncio.new_event_loop" title="asyncio.new_event_loop"><code class="xref py py-func docutils literal notranslate"><span class="pre">new_event_loop()</span></code></a> functions can be customized.</p> <p>Policy objects should implement the APIs defined in the <a class="reference internal" href="#asyncio.AbstractEventLoopPolicy" title="asyncio.AbstractEventLoopPolicy"><code class="xref py py-class docutils literal notranslate"><span class="pre">AbstractEventLoopPolicy</span></code></a> abstract base class.</p> <div class="section" id="getting-and-setting-the-policy"> <h2>Getting and Setting the Policy<a class="headerlink" href="#getting-and-setting-the-policy" title="Permalink to this headline">¶</a></h2> <p>The following functions can be used to get and set the policy for the current process:</p> <dl class="function"> <dt id="asyncio.get_event_loop_policy"> <code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">get_event_loop_policy</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.get_event_loop_policy" title="Permalink to this definition">¶</a></dt> <dd><p>Return the current process-wide policy.</p> </dd></dl> <dl class="function"> <dt id="asyncio.set_event_loop_policy"> <code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">set_event_loop_policy</code><span class="sig-paren">(</span><em class="sig-param">policy</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.set_event_loop_policy" title="Permalink to this definition">¶</a></dt> <dd><p>Set the current process-wide policy to <em>policy</em>.</p> <p>If <em>policy</em> is set to <code class="docutils literal notranslate"><span class="pre">None</span></code>, the default policy is restored.</p> </dd></dl> </div> <div class="section" id="policy-objects"> <h2>Policy Objects<a class="headerlink" href="#policy-objects" title="Permalink to this headline">¶</a></h2> <p>The abstract event loop policy base class is defined as follows:</p> <dl class="class"> <dt id="asyncio.AbstractEventLoopPolicy"> <em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">AbstractEventLoopPolicy</code><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy" title="Permalink to this definition">¶</a></dt> <dd><p>An abstract base class for asyncio policies.</p> <dl class="method"> <dt id="asyncio.AbstractEventLoopPolicy.get_event_loop"> <code class="sig-name descname">get_event_loop</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.get_event_loop" title="Permalink to this definition">¶</a></dt> <dd><p>Get the event loop for the current context.</p> <p>Return an event loop object implementing the <a class="reference internal" href="asyncio-eventloop.html#asyncio.AbstractEventLoop" title="asyncio.AbstractEventLoop"><code class="xref py py-class docutils literal notranslate"><span class="pre">AbstractEventLoop</span></code></a> interface.</p> <p>This method should never return <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.6.</span></p> </div> </dd></dl> <dl class="method"> <dt id="asyncio.AbstractEventLoopPolicy.set_event_loop"> <code class="sig-name descname">set_event_loop</code><span class="sig-paren">(</span><em class="sig-param">loop</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.set_event_loop" title="Permalink to this definition">¶</a></dt> <dd><p>Set the event loop for the current context to <em>loop</em>.</p> </dd></dl> <dl class="method"> <dt id="asyncio.AbstractEventLoopPolicy.new_event_loop"> <code class="sig-name descname">new_event_loop</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.new_event_loop" title="Permalink to this definition">¶</a></dt> <dd><p>Create and return a new event loop object.</p> <p>This method should never return <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p> </dd></dl> <dl class="method"> <dt id="asyncio.AbstractEventLoopPolicy.get_child_watcher"> <code class="sig-name descname">get_child_watcher</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.get_child_watcher" title="Permalink to this definition">¶</a></dt> <dd><p>Get a child process watcher object.</p> <p>Return a watcher object implementing the <a class="reference internal" href="#asyncio.AbstractChildWatcher" title="asyncio.AbstractChildWatcher"><code class="xref py py-class docutils literal notranslate"><span class="pre">AbstractChildWatcher</span></code></a> interface.</p> <p>This function is Unix specific.</p> </dd></dl> <dl class="method"> <dt id="asyncio.AbstractEventLoopPolicy.set_child_watcher"> <code class="sig-name descname">set_child_watcher</code><span class="sig-paren">(</span><em class="sig-param">watcher</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.set_child_watcher" title="Permalink to this definition">¶</a></dt> <dd><p>Set the current child process watcher to <em>watcher</em>.</p> <p>This function is Unix specific.</p> </dd></dl> </dd></dl> <p>asyncio ships with the following built-in policies:</p> <dl class="class"> <dt id="asyncio.DefaultEventLoopPolicy"> <em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">DefaultEventLoopPolicy</code><a class="headerlink" href="#asyncio.DefaultEventLoopPolicy" title="Permalink to this definition">¶</a></dt> <dd><p>The default asyncio policy. Uses <a class="reference internal" href="asyncio-eventloop.html#asyncio.SelectorEventLoop" title="asyncio.SelectorEventLoop"><code class="xref py py-class docutils literal notranslate"><span class="pre">SelectorEventLoop</span></code></a> on both Unix and Windows platforms.</p> <p>There is no need to install the default policy manually. asyncio is configured to use the default policy automatically.</p> </dd></dl> <dl class="class"> <dt id="asyncio.WindowsProactorEventLoopPolicy"> <em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">WindowsProactorEventLoopPolicy</code><a class="headerlink" href="#asyncio.WindowsProactorEventLoopPolicy" title="Permalink to this definition">¶</a></dt> <dd><p>An alternative event loop policy that uses the <a class="reference internal" href="asyncio-eventloop.html#asyncio.ProactorEventLoop" title="asyncio.ProactorEventLoop"><code class="xref py py-class docutils literal notranslate"><span class="pre">ProactorEventLoop</span></code></a> event loop implementation.</p> <p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p> </dd></dl> </div> <div class="section" id="process-watchers"> <h2>Process Watchers<a class="headerlink" href="#process-watchers" title="Permalink to this headline">¶</a></h2> <p>A process watcher allows customization of how an event loop monitors child processes on Unix. Specifically, the event loop needs to know when a child process has exited.</p> <p>In asyncio, child processes are created with <a class="reference internal" href="asyncio-subprocess.html#asyncio.create_subprocess_exec" title="asyncio.create_subprocess_exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">create_subprocess_exec()</span></code></a> and <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.subprocess_exec" title="asyncio.loop.subprocess_exec"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.subprocess_exec()</span></code></a> functions.</p> <p>asyncio defines the <a class="reference internal" href="#asyncio.AbstractChildWatcher" title="asyncio.AbstractChildWatcher"><code class="xref py py-class docutils literal notranslate"><span class="pre">AbstractChildWatcher</span></code></a> abstract base class, which child watchers should implement, and has two different implementations: <a class="reference internal" href="#asyncio.SafeChildWatcher" title="asyncio.SafeChildWatcher"><code class="xref py py-class docutils literal notranslate"><span class="pre">SafeChildWatcher</span></code></a> (configured to be used by default) and <a class="reference internal" href="#asyncio.FastChildWatcher" title="asyncio.FastChildWatcher"><code class="xref py py-class docutils literal notranslate"><span class="pre">FastChildWatcher</span></code></a>.</p> <p>See also the <a class="reference internal" href="asyncio-subprocess.html#asyncio-subprocess-threads"><span class="std std-ref">Subprocess and Threads</span></a> section.</p> <p>The following two functions can be used to customize the child process watcher implementation used by the asyncio event loop:</p> <dl class="function"> <dt id="asyncio.get_child_watcher"> <code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">get_child_watcher</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.get_child_watcher" title="Permalink to this definition">¶</a></dt> <dd><p>Return the current child watcher for the current policy.</p> </dd></dl> <dl class="function"> <dt id="asyncio.set_child_watcher"> <code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">set_child_watcher</code><span class="sig-paren">(</span><em class="sig-param">watcher</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.set_child_watcher" title="Permalink to this definition">¶</a></dt> <dd><p>Set the current child watcher to <em>watcher</em> for the current policy. <em>watcher</em> must implement methods defined in the <a class="reference internal" href="#asyncio.AbstractChildWatcher" title="asyncio.AbstractChildWatcher"><code class="xref py py-class docutils literal notranslate"><span class="pre">AbstractChildWatcher</span></code></a> base class.</p> </dd></dl> <div class="admonition note"> <p class="admonition-title">Note</p> <p>Third-party event loops implementations might not support custom child watchers. For such event loops, using <a class="reference internal" href="#asyncio.set_child_watcher" title="asyncio.set_child_watcher"><code class="xref py py-func docutils literal notranslate"><span class="pre">set_child_watcher()</span></code></a> might be prohibited or have no effect.</p> </div> <dl class="class"> <dt id="asyncio.AbstractChildWatcher"> <em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">AbstractChildWatcher</code><a class="headerlink" href="#asyncio.AbstractChildWatcher" title="Permalink to this definition">¶</a></dt> <dd><dl class="method"> <dt id="asyncio.AbstractChildWatcher.add_child_handler"> <code class="sig-name descname">add_child_handler</code><span class="sig-paren">(</span><em class="sig-param">pid</em>, <em class="sig-param">callback</em>, <em class="sig-param">*args</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.add_child_handler" title="Permalink to this definition">¶</a></dt> <dd><p>Register a new child handler.</p> <p>Arrange for <code class="docutils literal notranslate"><span class="pre">callback(pid,</span> <span class="pre">returncode,</span> <span class="pre">*args)</span></code> to be called when a process with PID equal to <em>pid</em> terminates. Specifying another callback for the same process replaces the previous handler.</p> <p>The <em>callback</em> callable must be thread-safe.</p> </dd></dl> <dl class="method"> <dt id="asyncio.AbstractChildWatcher.remove_child_handler"> <code class="sig-name descname">remove_child_handler</code><span class="sig-paren">(</span><em class="sig-param">pid</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.remove_child_handler" title="Permalink to this definition">¶</a></dt> <dd><p>Removes the handler for process with PID equal to <em>pid</em>.</p> <p>The function returns <code class="docutils literal notranslate"><span class="pre">True</span></code> if the handler was successfully removed, <code class="docutils literal notranslate"><span class="pre">False</span></code> if there was nothing to remove.</p> </dd></dl> <dl class="method"> <dt id="asyncio.AbstractChildWatcher.attach_loop"> <code class="sig-name descname">attach_loop</code><span class="sig-paren">(</span><em class="sig-param">loop</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.attach_loop" title="Permalink to this definition">¶</a></dt> <dd><p>Attach the watcher to an event loop.</p> <p>If the watcher was previously attached to an event loop, then it is first detached before attaching to the new loop.</p> <p>Note: loop may be <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p> </dd></dl> <dl class="method"> <dt id="asyncio.AbstractChildWatcher.close"> <code class="sig-name descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.close" title="Permalink to this definition">¶</a></dt> <dd><p>Close the watcher.</p> <p>This method has to be called to ensure that underlying resources are cleaned-up.</p> </dd></dl> </dd></dl> <dl class="class"> <dt id="asyncio.SafeChildWatcher"> <em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">SafeChildWatcher</code><a class="headerlink" href="#asyncio.SafeChildWatcher" title="Permalink to this definition">¶</a></dt> <dd><p>This implementation avoids disrupting other code spawning processes by polling every process explicitly on a <code class="xref py py-data docutils literal notranslate"><span class="pre">SIGCHLD</span></code> signal.</p> <p>This is a safe solution but it has a significant overhead when handling a big number of processes (<em>O(n)</em> each time a <code class="xref py py-data docutils literal notranslate"><span class="pre">SIGCHLD</span></code> is received).</p> <p>asyncio uses this safe implementation by default.</p> </dd></dl> <dl class="class"> <dt id="asyncio.FastChildWatcher"> <em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">FastChildWatcher</code><a class="headerlink" href="#asyncio.FastChildWatcher" title="Permalink to this definition">¶</a></dt> <dd><p>This implementation reaps every terminated processes by calling <code class="docutils literal notranslate"><span class="pre">os.waitpid(-1)</span></code> directly, possibly breaking other code spawning processes and waiting for their termination.</p> <p>There is no noticeable overhead when handling a big number of children (<em>O(1)</em> each time a child terminates).</p> </dd></dl> </div> <div class="section" id="custom-policies"> <h2>Custom Policies<a class="headerlink" href="#custom-policies" title="Permalink to this headline">¶</a></h2> <p>To implement a new event loop policy, it is recommended to subclass <a class="reference internal" href="#asyncio.DefaultEventLoopPolicy" title="asyncio.DefaultEventLoopPolicy"><code class="xref py py-class docutils literal notranslate"><span class="pre">DefaultEventLoopPolicy</span></code></a> and override the methods for which custom behavior is wanted, e.g.:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MyEventLoopPolicy</span><span class="p">(</span><span class="n">asyncio</span><span class="o">.</span><span class="n">DefaultEventLoopPolicy</span><span class="p">):</span> <span class="k">def</span> <span class="nf">get_event_loop</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="sd">"""Get the event loop.</span> <span class="sd"> This may be None or an instance of EventLoop.</span> <span class="sd"> """</span> <span class="n">loop</span> <span class="o">=</span> <span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="n">get_event_loop</span><span class="p">()</span> <span class="c1"># Do something with loop ...</span> <span class="k">return</span> <span class="n">loop</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">set_event_loop_policy</span><span class="p">(</span><span class="n">MyEventLoopPolicy</span><span class="p">())</span> </pre></div> </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="#">Policies</a><ul> <li><a class="reference internal" href="#getting-and-setting-the-policy">Getting and Setting the Policy</a></li> <li><a class="reference internal" href="#policy-objects">Policy Objects</a></li> <li><a class="reference internal" href="#process-watchers">Process Watchers</a></li> <li><a class="reference internal" href="#custom-policies">Custom Policies</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="asyncio-protocol.html" title="previous chapter">Transports and Protocols</a></p> <h4>Next topic</h4> <p class="topless"><a 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