<!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 href="asyncio-platforms.html"
title="next chapter">Platform Support</a></p>
<div role="note" aria-label="source link">
<h3>This Page</h3>
<ul class="this-page-menu">
<li><a href="../bugs.html">Report a Bug</a></li>
<li>
<a href="https://github.com/python/cpython/blob/3.7/Doc/library/asyncio-policy.rst"
rel="nofollow">Show Source
</a>
</li>
</ul>
</div>
</div>
</div>
<div class="clearer"></div>
</div>
<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"
>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"
>next</a> |</li>
<li class="right" >
<a href="asyncio-protocol.html" title="Transports and Protocols"
>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" ><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="footer">
© <a href="../copyright.html">Copyright</a> 2001-2021, Python Software Foundation.
<br />
The Python Software Foundation is a non-profit corporation.
<a href="https://www.python.org/psf/donations/">Please donate.</a>
<br />
Last updated on Feb 26, 2021.
<a href="../bugs.html">Found a bug</a>?
<br />
Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 2.3.1.
</div>
</body>
</html>
