new_event_loop在异步python3中的工作原理

Question

下面的代码摘自python3.8的asyncio包中的events.py。

Python使用new_event_loop创建新的循环，它返回self._loop_factory()，但是_loop_factory只是'NoneType‘对象是不可调用的，它是如何工作的？

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是一个基类。该属性由特定于平台的类(如从BaseDefaultEventLoopPolicy和make _loop_factory a class attribute that's something callable继承的_UnixDefaultEventLoopPolicy )设置为None以外的值

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

如果_loop_factory为None，则基类可以引发NotImplementedError，但作者可能希望避免该开销。