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.. currentmodule:: asyncio
.. _asyncio-policies:
========
Policies
========
.. warning::
Policies are deprecated and will be removed in Python 3.16.
Users are encouraged to use the :func:`asyncio.run` function
or the :class:`asyncio.Runner` with *loop_factory* to use
the desired loop implementation.
An event loop policy is a global object
used to get and set the current :ref:`event loop <asyncio-event-loop>`,
as well as create new event loops.
The default policy can be :ref:`replaced <asyncio-policy-get-set>` with
:ref:`built-in alternatives <asyncio-policy-builtin>`
to use different event loop implementations,
or substituted by a :ref:`custom policy <asyncio-custom-policies>`
that can override these behaviors.
The :ref:`policy object <asyncio-policy-objects>`
gets and sets a separate event loop per *context*.
This is per-thread by default,
though custom policies could define *context* differently.
Custom event loop policies can control the behavior of
:func:`get_event_loop`, :func:`set_event_loop`, and :func:`new_event_loop`.
Policy objects should implement the APIs defined
in the :class:`AbstractEventLoopPolicy` abstract base class.
.. _asyncio-policy-get-set:
Getting and Setting the Policy
==============================
The following functions can be used to get and set the policy
for the current process:
.. function:: get_event_loop_policy()
Return the current process-wide policy.
.. deprecated:: 3.14
The :func:`get_event_loop_policy` function is deprecated and
will be removed in Python 3.16.
.. function:: set_event_loop_policy(policy)
Set the current process-wide policy to *policy*.
If *policy* is set to ``None``, the default policy is restored.
.. deprecated:: 3.14
The :func:`set_event_loop_policy` function is deprecated and
will be removed in Python 3.16.
.. _asyncio-policy-objects:
Policy Objects
==============
The abstract event loop policy base class is defined as follows:
.. class:: AbstractEventLoopPolicy
An abstract base class for asyncio policies.
.. method:: get_event_loop()
Get the event loop for the current context.
Return an event loop object implementing the
:class:`AbstractEventLoop` interface.
This method should never return ``None``.
.. versionchanged:: 3.6
.. method:: set_event_loop(loop)
Set the event loop for the current context to *loop*.
.. method:: new_event_loop()
Create and return a new event loop object.
This method should never return ``None``.
.. deprecated:: 3.14
The :class:`AbstractEventLoopPolicy` class is deprecated and
will be removed in Python 3.16.
.. _asyncio-policy-builtin:
asyncio ships with the following built-in policies:
.. class:: DefaultEventLoopPolicy
The default asyncio policy. Uses :class:`SelectorEventLoop`
on Unix and :class:`ProactorEventLoop` on Windows.
There is no need to install the default policy manually. asyncio
is configured to use the default policy automatically.
.. versionchanged:: 3.8
On Windows, :class:`ProactorEventLoop` is now used by default.
.. versionchanged:: 3.14
The :meth:`get_event_loop` method of the default asyncio policy now
raises a :exc:`RuntimeError` if there is no set event loop.
.. deprecated:: 3.14
The :class:`DefaultEventLoopPolicy` class is deprecated and
will be removed in Python 3.16.
.. class:: WindowsSelectorEventLoopPolicy
An alternative event loop policy that uses the
:class:`SelectorEventLoop` event loop implementation.
.. availability:: Windows.
.. deprecated:: 3.14
The :class:`WindowsSelectorEventLoopPolicy` class is deprecated and
will be removed in Python 3.16.
.. class:: WindowsProactorEventLoopPolicy
An alternative event loop policy that uses the
:class:`ProactorEventLoop` event loop implementation.
.. availability:: Windows.
.. deprecated:: 3.14
The :class:`WindowsProactorEventLoopPolicy` class is deprecated and
will be removed in Python 3.16.
.. _asyncio-custom-policies:
Custom Policies
===============
To implement a new event loop policy, it is recommended to subclass
:class:`DefaultEventLoopPolicy` and override the methods for which
custom behavior is wanted, e.g.::
class MyEventLoopPolicy(asyncio.DefaultEventLoopPolicy):
def get_event_loop(self):
"""Get the event loop.
This may be None or an instance of EventLoop.
"""
loop = super().get_event_loop()
# Do something with loop ...
return loop
asyncio.set_event_loop_policy(MyEventLoopPolicy())
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