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asyncio
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# Policies {#asyncio-policies}
:::: warning
::: title
Warning
:::
Policies are deprecated and will be removed in Python 3.16. Users are encouraged to use the `asyncio.run`{.interpreted-text role="func"} function or the `asyncio.Runner`{.interpreted-text role="class"} with *loop_factory* to use the desired loop implementation.
::::
An event loop policy is a global object used to get and set the current `event loop <asyncio-event-loop>`{.interpreted-text role="ref"}, as well as create new event loops. The default policy can be `replaced <asyncio-policy-get-set>`{.interpreted-text role="ref"} with `built-in alternatives <asyncio-policy-builtin>`{.interpreted-text role="ref"} to use different event loop implementations, or substituted by a `custom policy <asyncio-custom-policies>`{.interpreted-text role="ref"} that can override these behaviors.
The `policy object <asyncio-policy-objects>`{.interpreted-text role="ref"} 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`{.interpreted-text role="func"}, `set_event_loop`{.interpreted-text role="func"}, and `new_event_loop`{.interpreted-text role="func"}.
Policy objects should implement the APIs defined in the `AbstractEventLoopPolicy`{.interpreted-text role="class"} abstract base class.
## Getting and Setting the Policy {#asyncio-policy-get-set}
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 `get_event_loop_policy`{.interpreted-text role="func"} 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 `set_event_loop_policy`{.interpreted-text role="func"} function is deprecated and will be removed in Python 3.16.
:::
::::
## Policy Objects {#asyncio-policy-objects}
The abstract event loop policy base class is defined as follows:
:::::::: AbstractEventLoopPolicy
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 `AbstractEventLoop`{.interpreted-text role="class"} 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 `AbstractEventLoopPolicy`{.interpreted-text role="class"} class is deprecated and will be removed in Python 3.16.
:::
::::::::
::: {#asyncio-policy-builtin}
asyncio ships with the following built-in policies:
:::
:::::: DefaultEventLoopPolicy
The default asyncio policy. Uses `SelectorEventLoop`{.interpreted-text role="class"} on Unix and `ProactorEventLoop`{.interpreted-text role="class"} 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, `ProactorEventLoop`{.interpreted-text role="class"} is now used by default.
:::
::: versionchanged
3.14 The `get_event_loop`{.interpreted-text role="meth"} method of the default asyncio policy now raises a `RuntimeError`{.interpreted-text role="exc"} if there is no set event loop.
:::
::: deprecated
3.14 The `DefaultEventLoopPolicy`{.interpreted-text role="class"} class is deprecated and will be removed in Python 3.16.
:::
::::::
::::: WindowsSelectorEventLoopPolicy
An alternative event loop policy that uses the `SelectorEventLoop`{.interpreted-text role="class"} event loop implementation.
::: availability
Windows.
:::
::: deprecated
3.14 The `WindowsSelectorEventLoopPolicy`{.interpreted-text role="class"} class is deprecated and will be removed in Python 3.16.
:::
:::::
::::: WindowsProactorEventLoopPolicy
An alternative event loop policy that uses the `ProactorEventLoop`{.interpreted-text role="class"} event loop implementation.
::: availability
Windows.
:::
::: deprecated
3.14 The `WindowsProactorEventLoopPolicy`{.interpreted-text role="class"} class is deprecated and will be removed in Python 3.16.
:::
:::::
## Custom Policies {#asyncio-custom-policies}
To implement a new event loop policy, it is recommended to subclass `DefaultEventLoopPolicy`{.interpreted-text role="class"} 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())
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