# 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`.

    Cambiato nella versione 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 and `ProactorEventLoop` on Windows.

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

  Cambiato nella versione 3.8: On Windows, `ProactorEventLoop` is now used by default.

  Deprecato dalla versione 3.12: The `get_event_loop()` method of the default asyncio policy now emits a `DeprecationWarning` 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())