Policies

Warning

Policies are deprecated and will be removed in Python 3.16.Users are encouraged to use theasyncio.run() functionor theasyncio.Runner withloop_factory to usethe desired loop implementation.

An event loop policy is a global objectused to get and set the currentevent loop,as well as create new event loops.The default policy can bereplaced withbuilt-in alternativesto use different event loop implementations,or substituted by acustom policythat can override these behaviors.

Thepolicy objectgets and sets a separate event loop percontext.This is per-thread by default,though custom policies could definecontext differently.

Custom event loop policies can control the behavior ofget_event_loop(),set_event_loop(), andnew_event_loop().

Policy objects should implement the APIs definedin theAbstractEventLoopPolicy abstract base class.

Getting and Setting the Policy

The following functions can be used to get and set the policyfor the current process:

asyncio.get_event_loop_policy()

Return the current process-wide policy.

Deprecated since version 3.14:Theget_event_loop_policy() function is deprecated andwill be removed in Python 3.16.

asyncio.set_event_loop_policy(policy)

Set the current process-wide policy topolicy.

Ifpolicy is set toNone, the default policy is restored.

Deprecated since version 3.14:Theset_event_loop_policy() function is deprecated andwill be removed in Python 3.16.

Policy Objects

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

classasyncio.AbstractEventLoopPolicy

An abstract base class for asyncio policies.

get_event_loop()

Get the event loop for the current context.

Return an event loop object implementing theAbstractEventLoop interface.

This method should never returnNone.

Changed in version 3.6.

set_event_loop(loop)

Set the event loop for the current context toloop.

new_event_loop()

Create and return a new event loop object.

This method should never returnNone.

Deprecated since version 3.14:TheAbstractEventLoopPolicy class is deprecated andwill be removed in Python 3.16.

asyncio ships with the following built-in policies:

classasyncio.DefaultEventLoopPolicy

The default asyncio policy. UsesSelectorEventLoopon Unix andProactorEventLoop on Windows.

There is no need to install the default policy manually. asynciois configured to use the default policy automatically.

Changed in version 3.8:On Windows,ProactorEventLoop is now used by default.

Changed in version 3.14:Theget_event_loop() method of the default asyncio policy nowraises aRuntimeError if there is no set event loop.

Deprecated since version 3.14:TheDefaultEventLoopPolicy class is deprecated andwill be removed in Python 3.16.

classasyncio.WindowsSelectorEventLoopPolicy

An alternative event loop policy that uses theSelectorEventLoop event loop implementation.

Availability: Windows.

Deprecated since version 3.14:TheWindowsSelectorEventLoopPolicy class is deprecated andwill be removed in Python 3.16.

classasyncio.WindowsProactorEventLoopPolicy

An alternative event loop policy that uses theProactorEventLoop event loop implementation.

Availability: Windows.

Deprecated since version 3.14:TheWindowsProactorEventLoopPolicy class is deprecated andwill be removed in Python 3.16.

Custom Policies

To implement a new event loop policy, it is recommended to subclassDefaultEventLoopPolicy and override the methods for whichcustom behavior is wanted, e.g.:

classMyEventLoopPolicy(asyncio.DefaultEventLoopPolicy):defget_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 ...returnloopasyncio.set_event_loop_policy(MyEventLoopPolicy())