18.5.2.1. Event loop functions

The following functions are convenient shortcuts to accessing the methods of the global policy. Note that this provides access to the default policy, unless an alternative policy was set by calling set_event_loop_policy() earlier in the execution of the process.

asyncio.get_event_loop()

Equivalent to calling get_event_loop_policy().get_event_loop().

asyncio.set_event_loop(loop)

Equivalent to calling get_event_loop_policy().set_event_loop(loop).

asyncio.new_event_loop()

Equivalent to calling get_event_loop_policy().new_event_loop().

18.5.2.2. Available event loops

asyncio currently provides two implementations of event loops: SelectorEventLoop and ProactorEventLoop.

class asyncio.SelectorEventLoop

Event loop based on the selectors module. Subclass of AbstractEventLoop.

Use the most efficient selector available on the platform.

On Windows, only sockets are supported (ex: pipes are not supported): see the MSDN documentation of select.

class asyncio.ProactorEventLoop

Proactor event loop for Windows using “I/O Completion Ports” aka IOCP. Subclass of AbstractEventLoop.

Availability: Windows.

See also

MSDN documentation on I/O Completion Ports.

Example to use a ProactorEventLoop on Windows:

import asyncio, sys

if sys.platform == 'win32':
    loop = asyncio.ProactorEventLoop()
    asyncio.set_event_loop(loop)

18.5.2.3. Platform support

The asyncio module has been designed to be portable, but each platform still has subtle differences and may not support all asyncio features.

import asyncio
import selectors

selector = selectors.SelectSelector()
loop = asyncio.SelectorEventLoop(selector)
asyncio.set_event_loop(loop)

18.5.2.4. Event loop policies and the default policy

Event loop management is abstracted with a policy pattern, to provide maximal flexibility for custom platforms and frameworks. Throughout the execution of a process, a single global policy object manages the event loops available to the process based on the calling context. A policy is an object implementing the AbstractEventLoopPolicy interface.

For most users of asyncio, policies never have to be dealt with explicitly, since the default global policy is sufficient (see below).

The module-level functions get_event_loop() and set_event_loop() provide convenient access to event loops managed by the default policy.

18.5.2.5. Event loop policy interface

An event loop policy must implement the following interface:

class asyncio.AbstractEventLoopPolicy

Event loop policy.

get_event_loop()

Get the event loop for the current context.

Returns an event loop object implementing the AbstractEventLoop interface.

Raises an exception in case no event loop has been set for the current context and the current policy does not specify to create one. It must never return None.

set_event_loop(loop)

Set the event loop for the current context to loop.

new_event_loop()

Create and return a new event loop object according to this policy’s rules.

If there’s need to set this loop as the event loop for the current context, set_event_loop() must be called explicitly.

The default policy defines context as the current thread, and manages an event loop per thread that interacts with asyncio. If the current thread doesn’t already have an event loop associated with it, the default policy’s get_event_loop() method creates one when called from the main thread, but raises RuntimeError otherwise.

18.5.2.6. Access to the global loop policy

asyncio.get_event_loop_policy()

Get the current event loop policy.

asyncio.set_event_loop_policy(policy)

Set the current event loop policy. If policy is None, the default policy is restored.

18.5.2.7. Customizing the event loop policy

To implement a new event loop policy, it is recommended you subclass the concrete default event loop policy DefaultEventLoopPolicy and override the methods for which you want to change behavior, for example:

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())