.. currentmodule:: asyncio

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.

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.

Policy Objects

The abstract event loop policy base class is defined as follows:

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 ships with the following built-in policies:

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.

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.

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.

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())