Policies¶
An event loop policy is a global object used to get and set the current event loop, as well as create new event loops. The default policy can be replaced with built-in alternatives to use different event loop implementations, or substituted by a custom policy that can override these behaviors.
The policy object 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
get_event_loop()
, set_event_loop()
, and new_event_loop()
.
Policy objects should implement the APIs defined
in the 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:
- asyncio.get_event_loop_policy()¶
Return the current process-wide policy.
- asyncio.set_event_loop_policy(policy)¶
Set the current process-wide policy to policy.
If policy is set to
None
, the default policy is restored.
Policy Objects¶
The abstract event loop policy base class is defined as follows:
- class asyncio.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 the
AbstractEventLoop
interface.This method should never return
None
.Changed in version 3.6.
- 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.
This method should never return
None
.
asyncio ships with the following built-in policies:
- class asyncio.DefaultEventLoopPolicy¶
The default asyncio policy. Uses
SelectorEventLoop
on Unix andProactorEventLoop
on Windows.There is no need to install the default policy manually. asyncio is configured to use the default policy automatically.
Changed in version 3.8: On Windows,
ProactorEventLoop
is now used by default.Deprecated since version 3.12: The
get_event_loop()
method of the default asyncio policy now emits aDeprecationWarning
if there is no current event loop set and it decides to create one. In some future Python release this will become an error.
- class asyncio.WindowsSelectorEventLoopPolicy¶
An alternative event loop policy that uses the
SelectorEventLoop
event loop implementation.Availability: Windows.
- class asyncio.WindowsProactorEventLoopPolicy¶
An alternative event loop policy that uses the
ProactorEventLoop
event loop implementation.Availability: Windows.
Custom Policies¶
To implement a new event loop policy, it is recommended to subclass
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())