python asynchronous context manager

问题

In Python Lan Ref. 3.4.4, it is said that __aenter__() and __aexit__() must return awaitables. However, in the example async context manager, these two methods return None:

class AsyncContextManager:
    async def __aenter__(self):
        await log('entering context')

    async def __aexit__(self, exc_type, exc, tb):
        await log('exiting context')

Is this code correct?

回答1:

Your __aenter__ method must return a context.

class MyAsyncContextManager:
    async def __aenter__(self):
        await log('entering context')
        # maybe some setup (e.g. await self.setup())
        return self

    async def __aexit__(self, exc_type, exc, tb):
        # maybe closing context (e.g. await self.close())
        await log('exiting context')

    async def do_something(self):
        await log('doing something')

usage:

async with MyAsyncContextManager() as context:
    await context.do_something()

回答2:

The methods in this example do not return None. They are async functions, which automatically return (awaitable) asynchronous coroutines. This is similar to how generator functions return generator iterators, even though they usually have no return statement.

来源：https://stackoverflow.com/questions/48390977/python-asynchronous-context-manager