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())
