# Policies

警告

Policies are deprecated and will be removed in Python 3.16. Users are encouraged to use the `asyncio.run()` function or the `asyncio.Runner` 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, 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.

  在 3.14 版之後被棄用: The `get_event_loop_policy()` function is deprecated and will be removed in Python 3.16.

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.

  在 3.14 版之後被棄用: The `set_event_loop_policy()` function is deprecated and will be removed in Python 3.16.

## 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`.

    在 3.6 版的變更.

  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`.

  在 3.14 版之後被棄用: The `AbstractEventLoopPolicy` class is deprecated and will be removed in Python 3.16.

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.

  在 3.8 版的變更: On Windows, `ProactorEventLoop` is now used by default.

  在 3.14 版的變更: The `get_event_loop()` method of the default asyncio policy now raises a `RuntimeError` if there is no set event loop.

  在 3.14 版之後被棄用: The `DefaultEventLoopPolicy` class is deprecated and will be removed in Python 3.16.

class asyncio.WindowsSelectorEventLoopPolicy

  An alternative event loop policy that uses the `SelectorEventLoop` event loop implementation.

  可用性: Windows.

  在 3.14 版之後被棄用: The `WindowsSelectorEventLoopPolicy` class is deprecated and will be removed in Python 3.16.

class asyncio.WindowsProactorEventLoopPolicy

  An alternative event loop policy that uses the `ProactorEventLoop` event loop implementation.

  可用性: Windows.

  在 3.14 版之後被棄用: The `WindowsProactorEventLoopPolicy` class is deprecated and will be removed in Python 3.16.

## 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())