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<div class="section" id="policies">
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<span id="asyncio-policies"></span><h1>Policies<a class="headerlink" href="#policies" title="Permalink to this headline">¶</a></h1>
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<p>An event loop policy is a global per-process object that controls
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the management of the event loop. Each event loop has a default
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policy, which can be changed and customized using the policy API.</p>
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<p>A policy defines the notion of <em>context</em> and manages a
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separate event loop per context. The default policy
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defines <em>context</em> to be the current thread.</p>
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<p>By using a custom event loop policy, the behavior of
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<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
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<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>
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<p>Policy objects should implement the APIs defined
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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>
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<div class="section" id="getting-and-setting-the-policy">
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<h2>Getting and Setting the Policy<a class="headerlink" href="#getting-and-setting-the-policy" title="Permalink to this headline">¶</a></h2>
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<p>The following functions can be used to get and set the policy
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for the current process:</p>
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<dl class="function">
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<dt id="asyncio.get_event_loop_policy">
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<code class="descclassname">asyncio.</code><code class="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>
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<dd><p>Return the current process-wide policy.</p>
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</dd></dl>
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<dl class="function">
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<dt id="asyncio.set_event_loop_policy">
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<code class="descclassname">asyncio.</code><code class="descname">set_event_loop_policy</code><span class="sig-paren">(</span><em>policy</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.set_event_loop_policy" title="Permalink to this definition">¶</a></dt>
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<dd><p>Set the current process-wide policy to <em>policy</em>.</p>
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<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>
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</dd></dl>
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</div>
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<div class="section" id="policy-objects">
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<h2>Policy Objects<a class="headerlink" href="#policy-objects" title="Permalink to this headline">¶</a></h2>
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<p>The abstract event loop policy base class is defined as follows:</p>
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<dl class="class">
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<dt id="asyncio.AbstractEventLoopPolicy">
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<em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">AbstractEventLoopPolicy</code><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy" title="Permalink to this definition">¶</a></dt>
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<dd><p>An abstract base class for asyncio policies.</p>
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<dl class="method">
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<dt id="asyncio.AbstractEventLoopPolicy.get_event_loop">
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<code class="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>
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<dd><p>Get the event loop for the current context.</p>
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<p>Return an event loop object implementing the
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<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>
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<p>This method should never return <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>
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<div class="versionchanged">
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<p><span class="versionmodified changed">Changed in version 3.6.</span></p>
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</div>
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</dd></dl>
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<dl class="method">
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<dt id="asyncio.AbstractEventLoopPolicy.set_event_loop">
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<code class="descname">set_event_loop</code><span class="sig-paren">(</span><em>loop</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.set_event_loop" title="Permalink to this definition">¶</a></dt>
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<dd><p>Set the event loop for the current context to <em>loop</em>.</p>
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</dd></dl>
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<dl class="method">
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<dt id="asyncio.AbstractEventLoopPolicy.new_event_loop">
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<code class="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>
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<dd><p>Create and return a new event loop object.</p>
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<p>This method should never return <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>
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</dd></dl>
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<dl class="method">
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<dt id="asyncio.AbstractEventLoopPolicy.get_child_watcher">
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<code class="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>
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<dd><p>Get a child process watcher object.</p>
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<p>Return a watcher object implementing the
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<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>
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<p>This function is Unix specific.</p>
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</dd></dl>
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<dl class="method">
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<dt id="asyncio.AbstractEventLoopPolicy.set_child_watcher">
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<code class="descname">set_child_watcher</code><span class="sig-paren">(</span><em>watcher</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.set_child_watcher" title="Permalink to this definition">¶</a></dt>
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<dd><p>Set the current child process watcher to <em>watcher</em>.</p>
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<p>This function is Unix specific.</p>
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</dd></dl>
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</dd></dl>
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<p>asyncio ships with the following built-in policies:</p>
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<dl class="class">
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<dt id="asyncio.DefaultEventLoopPolicy">
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<em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">DefaultEventLoopPolicy</code><a class="headerlink" href="#asyncio.DefaultEventLoopPolicy" title="Permalink to this definition">¶</a></dt>
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<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>
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on both Unix and Windows platforms.</p>
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<p>There is no need to install the default policy manually. asyncio
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is configured to use the default policy automatically.</p>
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</dd></dl>
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<dl class="class">
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<dt id="asyncio.WindowsProactorEventLoopPolicy">
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<em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">WindowsProactorEventLoopPolicy</code><a class="headerlink" href="#asyncio.WindowsProactorEventLoopPolicy" title="Permalink to this definition">¶</a></dt>
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<dd><p>An alternative event loop policy that uses the
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<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>
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<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
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</dd></dl>
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</div>
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<div class="section" id="process-watchers">
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<h2>Process Watchers<a class="headerlink" href="#process-watchers" title="Permalink to this headline">¶</a></h2>
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<p>A process watcher allows customization of how an event loop monitors
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child processes on Unix. Specifically, the event loop needs to know
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when a child process has exited.</p>
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<p>In asyncio, child processes are created with
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<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>
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functions.</p>
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<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,
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which child watchers should implement, and has two different
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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
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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>
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<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>
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section.</p>
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<p>The following two functions can be used to customize the child process watcher
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implementation used by the asyncio event loop:</p>
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<dl class="function">
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||
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<dt id="asyncio.get_child_watcher">
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<code class="descclassname">asyncio.</code><code class="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>
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<dd><p>Return the current child watcher for the current policy.</p>
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</dd></dl>
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<dl class="function">
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<dt id="asyncio.set_child_watcher">
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<code class="descclassname">asyncio.</code><code class="descname">set_child_watcher</code><span class="sig-paren">(</span><em>watcher</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.set_child_watcher" title="Permalink to this definition">¶</a></dt>
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<dd><p>Set the current child watcher to <em>watcher</em> for the current
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policy. <em>watcher</em> must implement methods defined in the
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<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>
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</dd></dl>
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<div class="admonition note">
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<p class="admonition-title">Note</p>
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<p>Third-party event loops implementations might not support
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custom child watchers. For such event loops, using
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<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>
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</div>
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<dl class="class">
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<dt id="asyncio.AbstractChildWatcher">
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<em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">AbstractChildWatcher</code><a class="headerlink" href="#asyncio.AbstractChildWatcher" title="Permalink to this definition">¶</a></dt>
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<dd><dl class="method">
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<dt id="asyncio.AbstractChildWatcher.add_child_handler">
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<code class="descname">add_child_handler</code><span class="sig-paren">(</span><em>pid</em>, <em>callback</em>, <em>*args</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.add_child_handler" title="Permalink to this definition">¶</a></dt>
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<dd><p>Register a new child handler.</p>
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<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
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when a process with PID equal to <em>pid</em> terminates. Specifying
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another callback for the same process replaces the previous
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handler.</p>
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|
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<p>The <em>callback</em> callable must be thread-safe.</p>
|
||
|
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</dd></dl>
|
||
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|
||
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<dl class="method">
|
||
|
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<dt id="asyncio.AbstractChildWatcher.remove_child_handler">
|
||
|
|
<code class="descname">remove_child_handler</code><span class="sig-paren">(</span><em>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>
|
||
|
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</dd></dl>
|
||
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|
||
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<dl class="method">
|
||
|
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<dt id="asyncio.AbstractChildWatcher.attach_loop">
|
||
|
|
<code class="descname">attach_loop</code><span class="sig-paren">(</span><em>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>
|
||
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|
||
|
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<dl class="method">
|
||
|
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<dt id="asyncio.AbstractChildWatcher.close">
|
||
|
|
<code class="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>
|
||
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|
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|
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</dd></dl>
|
||
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|
||
|
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<dl class="class">
|
||
|
|
<dt id="asyncio.SafeChildWatcher">
|
||
|
|
<em class="property">class </em><code class="descclassname">asyncio.</code><code class="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="descclassname">asyncio.</code><code class="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>
|
||
|
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</div>
|
||
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</div>
|
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<h3><a href="../contents.html">Table of Contents</a></h3>
|
||
|
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<ul>
|
||
|
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<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>
|
||
|
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<li><a class="reference internal" href="#custom-policies">Custom Policies</a></li>
|
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|
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