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<li><a class="reference internal" href="#">Policies</a><ul>
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<li><a class="reference internal" href="#getting-and-setting-the-policy">Getting and Setting the Policy</a></li>
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<li><a class="reference internal" href="#policy-objects">Policy Objects</a></li>
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<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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<section id="policies">
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<span id="asyncio-policies"></span><h1>Policies<a class="headerlink" href="#policies" title="Link to this heading">¶</a></h1>
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<p>An event loop policy is a global object
|
||
used to get and set the current <a class="reference internal" href="asyncio-eventloop.html#asyncio-event-loop"><span class="std std-ref">event loop</span></a>,
|
||
as well as create new event loops.
|
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The default policy can be <a class="reference internal" href="#asyncio-policy-get-set"><span class="std std-ref">replaced</span></a> with
|
||
<a class="reference internal" href="#asyncio-policy-builtin"><span class="std std-ref">built-in alternatives</span></a>
|
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to use different event loop implementations,
|
||
or substituted by a <a class="reference internal" href="#asyncio-custom-policies"><span class="std std-ref">custom policy</span></a>
|
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that can override these behaviors.</p>
|
||
<p>The <a class="reference internal" href="#asyncio-policy-objects"><span class="std std-ref">policy object</span></a>
|
||
gets and sets a separate event loop per <em>context</em>.
|
||
This is per-thread by default,
|
||
though custom policies could define <em>context</em> differently.</p>
|
||
<p>Custom event loop policies can control 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>.</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>
|
||
<section id="getting-and-setting-the-policy">
|
||
<span id="asyncio-policy-get-set"></span><h2>Getting and Setting the Policy<a class="headerlink" href="#getting-and-setting-the-policy" title="Link to this heading">¶</a></h2>
|
||
<p>The following functions can be used to get and set the policy
|
||
for the current process:</p>
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="asyncio.get_event_loop_policy">
|
||
<span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">get_event_loop_policy</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.get_event_loop_policy" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Return the current process-wide policy.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="asyncio.set_event_loop_policy">
|
||
<span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">set_event_loop_policy</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">policy</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.set_event_loop_policy" title="Link 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>
|
||
|
||
</section>
|
||
<section id="policy-objects">
|
||
<span id="asyncio-policy-objects"></span><h2>Policy Objects<a class="headerlink" href="#policy-objects" title="Link to this heading">¶</a></h2>
|
||
<p>The abstract event loop policy base class is defined as follows:</p>
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractEventLoopPolicy">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">AbstractEventLoopPolicy</span></span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy" title="Link to this definition">¶</a></dt>
|
||
<dd><p>An abstract base class for asyncio policies.</p>
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractEventLoopPolicy.get_event_loop">
|
||
<span class="sig-name descname"><span class="pre">get_event_loop</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.get_event_loop" title="Link 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="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractEventLoopPolicy.set_event_loop">
|
||
<span class="sig-name descname"><span class="pre">set_event_loop</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">loop</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.set_event_loop" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Set the event loop for the current context to <em>loop</em>.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractEventLoopPolicy.new_event_loop">
|
||
<span class="sig-name descname"><span class="pre">new_event_loop</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.new_event_loop" title="Link 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="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractEventLoopPolicy.get_child_watcher">
|
||
<span class="sig-name descname"><span class="pre">get_child_watcher</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.get_child_watcher" title="Link 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>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">Deprecated since version 3.12.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractEventLoopPolicy.set_child_watcher">
|
||
<span class="sig-name descname"><span class="pre">set_child_watcher</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">watcher</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractEventLoopPolicy.set_child_watcher" title="Link 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>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">Deprecated since version 3.12.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
</dd></dl>
|
||
|
||
<p id="asyncio-policy-builtin">asyncio ships with the following built-in policies:</p>
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="asyncio.DefaultEventLoopPolicy">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">DefaultEventLoopPolicy</span></span><a class="headerlink" href="#asyncio.DefaultEventLoopPolicy" title="Link 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 Unix and <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> on Windows.</p>
|
||
<p>There is no need to install the default policy manually. asyncio
|
||
is configured to use the default policy automatically.</p>
|
||
<div class="versionchanged">
|
||
<p><span class="versionmodified changed">Changed in version 3.8: </span>On Windows, <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> is now used by default.</p>
|
||
</div>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">Deprecated since version 3.12: </span>The <a class="reference internal" href="asyncio-eventloop.html#asyncio.get_event_loop" title="asyncio.get_event_loop"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get_event_loop()</span></code></a> method of the default asyncio policy now emits
|
||
a <a class="reference internal" href="exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">DeprecationWarning</span></code></a> if there is no current event loop set and it
|
||
decides to create one.
|
||
In some future Python release this will become an error.</p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="asyncio.WindowsSelectorEventLoopPolicy">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">WindowsSelectorEventLoopPolicy</span></span><a class="headerlink" href="#asyncio.WindowsSelectorEventLoopPolicy" title="Link to this definition">¶</a></dt>
|
||
<dd><p>An alternative event loop policy that uses the
|
||
<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> event loop implementation.</p>
|
||
<div class="availability docutils container">
|
||
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="asyncio.WindowsProactorEventLoopPolicy">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">WindowsProactorEventLoopPolicy</span></span><a class="headerlink" href="#asyncio.WindowsProactorEventLoopPolicy" title="Link 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>
|
||
<div class="availability docutils container">
|
||
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
</section>
|
||
<section id="process-watchers">
|
||
<span id="asyncio-watchers"></span><h2>Process Watchers<a class="headerlink" href="#process-watchers" title="Link to this heading">¶</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 four different implementations:
|
||
<a class="reference internal" href="#asyncio.ThreadedChildWatcher" title="asyncio.ThreadedChildWatcher"><code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadedChildWatcher</span></code></a> (configured to be used by default),
|
||
<a class="reference internal" href="#asyncio.MultiLoopChildWatcher" title="asyncio.MultiLoopChildWatcher"><code class="xref py py-class docutils literal notranslate"><span class="pre">MultiLoopChildWatcher</span></code></a>, <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>, 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="py function">
|
||
<dt class="sig sig-object py" id="asyncio.get_child_watcher">
|
||
<span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">get_child_watcher</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.get_child_watcher" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Return the current child watcher for the current policy.</p>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">Deprecated since version 3.12.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="asyncio.set_child_watcher">
|
||
<span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">set_child_watcher</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">watcher</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.set_child_watcher" title="Link 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>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">Deprecated since version 3.12.</span></p>
|
||
</div>
|
||
</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="py class">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractChildWatcher">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">AbstractChildWatcher</span></span><a class="headerlink" href="#asyncio.AbstractChildWatcher" title="Link to this definition">¶</a></dt>
|
||
<dd><dl class="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractChildWatcher.add_child_handler">
|
||
<span class="sig-name descname"><span class="pre">add_child_handler</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">pid</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">callback</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span><span class="n"><span class="pre">args</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.add_child_handler" title="Link 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="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractChildWatcher.remove_child_handler">
|
||
<span class="sig-name descname"><span class="pre">remove_child_handler</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">pid</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.remove_child_handler" title="Link 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="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractChildWatcher.attach_loop">
|
||
<span class="sig-name descname"><span class="pre">attach_loop</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">loop</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.attach_loop" title="Link 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="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractChildWatcher.is_active">
|
||
<span class="sig-name descname"><span class="pre">is_active</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.is_active" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the watcher is ready to use.</p>
|
||
<p>Spawning a subprocess with <em>inactive</em> current child watcher raises
|
||
<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>.</p>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">Added in version 3.8.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="asyncio.AbstractChildWatcher.close">
|
||
<span class="sig-name descname"><span class="pre">close</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.AbstractChildWatcher.close" title="Link 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>
|
||
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">Deprecated since version 3.12.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="asyncio.ThreadedChildWatcher">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">ThreadedChildWatcher</span></span><a class="headerlink" href="#asyncio.ThreadedChildWatcher" title="Link to this definition">¶</a></dt>
|
||
<dd><p>This implementation starts a new waiting thread for every subprocess spawn.</p>
|
||
<p>It works reliably even when the asyncio event loop is run in a non-main OS thread.</p>
|
||
<p>There is no noticeable overhead when handling a big number of children (<em>O</em>(1) each
|
||
time a child terminates), but starting a thread per process requires extra memory.</p>
|
||
<p>This watcher is used by default.</p>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">Added in version 3.8.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="asyncio.MultiLoopChildWatcher">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">MultiLoopChildWatcher</span></span><a class="headerlink" href="#asyncio.MultiLoopChildWatcher" title="Link to this definition">¶</a></dt>
|
||
<dd><p>This implementation registers a <code class="xref py py-data docutils literal notranslate"><span class="pre">SIGCHLD</span></code> signal handler on
|
||
instantiation. That can break third-party code that installs a custom handler for
|
||
<code class="xref py py-data docutils literal notranslate"><span class="pre">SIGCHLD</span></code> signal.</p>
|
||
<p>The watcher 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>There is no limitation for running subprocesses from different threads once the
|
||
watcher is installed.</p>
|
||
<p>The solution is safe but it has a significant overhead when
|
||
handling a big number of processes (<em>O</em>(<em>n</em>) each time a
|
||
<code class="xref py py-data docutils literal notranslate"><span class="pre">SIGCHLD</span></code> is received).</p>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">Added in version 3.8.</span></p>
|
||
</div>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">Deprecated since version 3.12.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="asyncio.SafeChildWatcher">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">SafeChildWatcher</span></span><a class="headerlink" href="#asyncio.SafeChildWatcher" title="Link to this definition">¶</a></dt>
|
||
<dd><p>This implementation uses active event loop from the main thread to handle
|
||
<code class="xref py py-data docutils literal notranslate"><span class="pre">SIGCHLD</span></code> signal. If the main thread has no running event loop another
|
||
thread cannot spawn a subprocess (<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>The watcher 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 solution is as safe as <a class="reference internal" href="#asyncio.MultiLoopChildWatcher" title="asyncio.MultiLoopChildWatcher"><code class="xref py py-class docutils literal notranslate"><span class="pre">MultiLoopChildWatcher</span></code></a> and has the same <em>O</em>(<em>n</em>)
|
||
complexity but requires a running event loop in the main thread to work.</p>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">Deprecated since version 3.12.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="asyncio.FastChildWatcher">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">FastChildWatcher</span></span><a class="headerlink" href="#asyncio.FastChildWatcher" title="Link 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</em>(1) each time a child terminates).</p>
|
||
<p>This solution requires a running event loop in the main thread to work, as
|
||
<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>.</p>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">Deprecated since version 3.12.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="asyncio.PidfdChildWatcher">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">PidfdChildWatcher</span></span><a class="headerlink" href="#asyncio.PidfdChildWatcher" title="Link to this definition">¶</a></dt>
|
||
<dd><p>This implementation polls process file descriptors (pidfds) to await child
|
||
process termination. In some respects, <a class="reference internal" href="#asyncio.PidfdChildWatcher" title="asyncio.PidfdChildWatcher"><code class="xref py py-class docutils literal notranslate"><span class="pre">PidfdChildWatcher</span></code></a> is a
|
||
“Goldilocks” child watcher implementation. It doesn’t require signals or
|
||
threads, doesn’t interfere with any processes launched outside the event
|
||
loop, and scales linearly with the number of subprocesses launched by the
|
||
event loop. The main disadvantage is that pidfds are specific to Linux, and
|
||
only work on recent (5.3+) kernels.</p>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">Added in version 3.9.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
</section>
|
||
<section id="custom-policies">
|
||
<span id="asyncio-custom-policies"></span><h2>Custom Policies<a class="headerlink" href="#custom-policies" title="Link to this heading">¶</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="w"> </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="w"> </span><span class="nf">get_event_loop</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="w"> </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>
|
||
</section>
|
||
</section>
|
||
|
||
|
||
<div class="clearer"></div>
|
||
</div>
|
||
</div>
|
||
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|
||
<div class="sphinxsidebar" role="navigation" aria-label="Main">
|
||
<div class="sphinxsidebarwrapper">
|
||
<div>
|
||
<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>
|
||
|
||
</div>
|
||
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|
||
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|
||
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||
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