Using client sessions without a context manager

[aliceh75]

It would be useful to be able to use ClientSession without a context manager.

Looking at the code:

• ClientSession.__aenter__ does not do anything (See https://github.com/aio-libs/aiohttp/blob/master/aiohttp/client.py#L827 );
• ClientSession.__aexit__ only does await self.close() (See https://github.com/aio-libs/aiohttp/blob/master/aiohttp/client.py#L830).

So the following currently works:

    session = ClientSession()
    async with session.get('...'):
        # ...
    await session.close()

However the documentation doesn't specify whether we can rely on this behaviour. I can see three options:

1. Make the current behaviour official by documenting it;
2. Add a different way to use ClientSession without a context manager (I would guess the main change would be to add an async ClientSession.start() even if it currently does nothing for future-proofing);
3. Do nothing, and document the fact people shouldn't rely on the current behaviour (people can always call __aenter__ and __aexit__ directly, I understand it's an accepted thing to do).

I'm happy to help and do a PR for whichever solution you prefer.

[asvetlov]

@aliceh75 sorry for delay.
Your snippet is correct, it is the official way for working with ClientSession without async context manager.
I thought it is obvious but looks like we need to document it explicitly.
Please feel free to make a PR with docs update.

The only neat: client = aiohttp.ClientSession() should be called from an async function, the session creation without active (running) event loop raises a user warning.