mirror of https://github.com/python/cpython
142 lines
3.6 KiB
ReStructuredText
142 lines
3.6 KiB
ReStructuredText
.. currentmodule:: asyncio
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.. _asyncio-policies:
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========
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Policies
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========
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An event loop policy is a global object
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used to get and set the current :ref:`event loop <asyncio-event-loop>`,
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as well as create new event loops.
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The default policy can be :ref:`replaced <asyncio-policy-get-set>` with
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:ref:`built-in alternatives <asyncio-policy-builtin>`
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to use different event loop implementations,
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or substituted by a :ref:`custom policy <asyncio-custom-policies>`
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that can override these behaviors.
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The :ref:`policy object <asyncio-policy-objects>`
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gets and sets a separate event loop per *context*.
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This is per-thread by default,
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though custom policies could define *context* differently.
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Custom event loop policies can control the behavior of
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:func:`get_event_loop`, :func:`set_event_loop`, and :func:`new_event_loop`.
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Policy objects should implement the APIs defined
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in the :class:`AbstractEventLoopPolicy` abstract base class.
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.. _asyncio-policy-get-set:
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Getting and Setting the Policy
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==============================
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The following functions can be used to get and set the policy
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for the current process:
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.. function:: get_event_loop_policy()
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Return the current process-wide policy.
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.. function:: set_event_loop_policy(policy)
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Set the current process-wide policy to *policy*.
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If *policy* is set to ``None``, the default policy is restored.
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.. _asyncio-policy-objects:
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Policy Objects
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==============
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The abstract event loop policy base class is defined as follows:
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.. class:: AbstractEventLoopPolicy
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An abstract base class for asyncio policies.
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.. method:: get_event_loop()
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Get the event loop for the current context.
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Return an event loop object implementing the
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:class:`AbstractEventLoop` interface.
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This method should never return ``None``.
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.. versionchanged:: 3.6
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.. method:: set_event_loop(loop)
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Set the event loop for the current context to *loop*.
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.. method:: new_event_loop()
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Create and return a new event loop object.
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This method should never return ``None``.
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.. _asyncio-policy-builtin:
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asyncio ships with the following built-in policies:
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.. class:: DefaultEventLoopPolicy
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The default asyncio policy. Uses :class:`SelectorEventLoop`
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on Unix and :class:`ProactorEventLoop` on Windows.
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There is no need to install the default policy manually. asyncio
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is configured to use the default policy automatically.
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.. versionchanged:: 3.8
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On Windows, :class:`ProactorEventLoop` is now used by default.
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.. versionchanged:: 3.14
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The :meth:`get_event_loop` method of the default asyncio policy now
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raises a :exc:`RuntimeError` if there is no set event loop.
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.. class:: WindowsSelectorEventLoopPolicy
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An alternative event loop policy that uses the
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:class:`SelectorEventLoop` event loop implementation.
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.. availability:: Windows.
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.. class:: WindowsProactorEventLoopPolicy
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An alternative event loop policy that uses the
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:class:`ProactorEventLoop` event loop implementation.
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.. availability:: Windows.
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.. _asyncio-custom-policies:
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Custom Policies
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===============
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To implement a new event loop policy, it is recommended to subclass
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:class:`DefaultEventLoopPolicy` and override the methods for which
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custom behavior is wanted, e.g.::
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class MyEventLoopPolicy(asyncio.DefaultEventLoopPolicy):
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def get_event_loop(self):
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"""Get the event loop.
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This may be None or an instance of EventLoop.
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"""
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loop = super().get_event_loop()
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# Do something with loop ...
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return loop
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asyncio.set_event_loop_policy(MyEventLoopPolicy())
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