new_event_loop 如何在 asyncio python3 中工作

Question

The code below is excerpted from events.py in package asyncio from python3.8.

Python use new_event_loop to create new loop, it return self._loop_factory(), but _loop_factory is just 'NoneType' object is not callable, how it works?

class BaseDefaultEventLoopPolicy(AbstractEventLoopPolicy):

    _loop_factory = None

    class _Local(threading.local):
        _loop = None
        _set_called = False

    def __init__(self):
        self._local = self._Local()

    def get_event_loop(self):
        """Get the event loop for the current context.

        Returns an instance of EventLoop or raises an exception.
        """
        if (self._local._loop is None and
                not self._local._set_called and
                isinstance(threading.current_thread(), threading._MainThread)):
            self.set_event_loop(self.new_event_loop())

        if self._local._loop is None:
            raise RuntimeError('There is no current event loop in thread %r.'
                               % threading.current_thread().name)

        return self._local._loop

    def set_event_loop(self, loop):
        """Set the event loop."""
        self._local._set_called = True
        assert loop is None or isinstance(loop, AbstractEventLoop)
        self._local._loop = loop

    def new_event_loop(self):
        """Create a new event loop.

        You must call set_event_loop() to make this the current event
        loop.
        """
        return self._loop_factory()

Answer 1

BaseDefaultEventLoopPolicy is a base class. That attribute is set to something other than None by the platform-specific classes such as _UnixDefaultEventLoopPolicy , which inherit from BaseDefaultEventLoopPolicy and make _loop_factory a class attribute that's something callable :

class _UnixDefaultEventLoopPolicy(events.BaseDefaultEventLoopPolicy):
    """UNIX event loop policy with a watcher for child processes."""
    _loop_factory = _UnixSelectorEventLoop

The base class could raise a NotImplementedError if _loop_factory is None, but it's possible that the authors wanted to avoid that overhead.