18.5.2.1.Event loop functions¶

The following functions are convenient shortcuts to accessing the methods of theglobal policy. Note that this provides access to the default policy, unless analternative policy was set by callingset_event_loop_policy() earlier inthe execution of the process.

asyncio.get_event_loop()¶

Equivalent to callingget_event_loop_policy().get_event_loop().

asyncio.set_event_loop(loop)¶

Equivalent to callingget_event_loop_policy().set_event_loop(loop).

asyncio.new_event_loop()¶

Equivalent to callingget_event_loop_policy().new_event_loop().

18.5.2.2.Available event loops¶

asyncio currently provides two implementations of event loops:SelectorEventLoop andProactorEventLoop.

classasyncio.SelectorEventLoop¶

Event loop based on theselectors module. Subclass ofAbstractEventLoop.

Use the most efficient selector available on the platform.

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

classasyncio.ProactorEventLoop¶

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

Availability: Windows.

See also

MSDN documentation on I/O Completion Ports.

Example to use aProactorEventLoop on Windows:

importasyncio,sysifsys.platform=='win32':loop=asyncio.ProactorEventLoop()asyncio.set_event_loop(loop)

18.5.2.3.Platform support¶

Theasyncio module has been designed to be portable, but each platformstill has subtle differences and may not support allasyncio features.

importasyncioimportselectorsselector=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 apolicy pattern, to provide maximalflexibility for custom platforms and frameworks. Throughout the execution of aprocess, a single global policy object manages the event loops available to theprocess based on the calling context. A policy is an object implementing theAbstractEventLoopPolicy interface.

For most users ofasyncio, policies never have to be dealt withexplicitly, since the default global policy is sufficient (see below).

The module-level functionsget_event_loop() andset_event_loop() provide convenient access toevent loops managed by the default policy.

18.5.2.5.Event loop policy interface¶

An event loop policy must implement the following interface:

classasyncio.AbstractEventLoopPolicy¶

Event loop policy.

get_event_loop()¶

Get the event loop for the current context.

Returns an event loop object implementing theAbstractEventLoopinterface. In case called from coroutine, it returns the currentlyrunning event loop.

Raises an exception in case no event loop has been set for the currentcontext and the current policy does not specify to create one. It mustnever 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 according to this policy’srules.

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

The default policy defines context as the current thread, and manages an eventloop per thread that interacts withasyncio. If the current threaddoesn’t already have an event loop associated with it, the default policy’sget_event_loop() method creates one whencalled from the main thread, but raisesRuntimeError 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. Ifpolicy isNone, the defaultpolicy is restored.

18.5.2.7.Customizing the event loop policy¶

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

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())