[docs] [issue30831] Inconsistent or wrong documentation around Asynchronous Context Manager

[Daisuke Miyakawa]

New submission from Daisuke Miyakawa:

I was reading the following doc and got confused.

https://docs.python.org/3.7/reference/datamodel.html#object.__aenter__

According to the API doc itself (and original PEP 492), __aenter__() "is semantically similar to the __enter__(), with only difference that it must return an awaitable." __aexit__() has similar sentence that confuses me too.

PEP 492 also implies the (awaitable) object returned from obj.__aenter__() will be awaited immediately. In same manner, the (awaitable) object returned from obj.__aexit__() will be awaited too.

(From PEP 492)
> async with EXPR as VAR:
>     BLOCK
>
> which is semantically equivalent to:
> 
> mgr = (EXPR)
> aexit = type(mgr).__aexit__
> aenter = type(mgr).__aenter__(mgr)
> exc = True
> 
> VAR = await aenter
> try:
>     BLOCK
> except:
>     if not await aexit(mgr, *sys.exc_info()):
>         raise
> else:
>     await aexit(mgr, None, None, None)

On the other hand, actual CPython implementation won't do that; it won't await the returned objects.

Moreover, the example shown in the API doc (AsyncContextManager) does NOT return awaitable as the doc itself suggests, but just await inside __aenter__() (and __aexit__()).

> class AsyncContextManager:
>    async def __aenter__(self):
>        await log('entering context')
>
>    async def __aexit__(self, exc_type, exc, tb):
>        await log('exiting context')

I'm not sure which is true; the doc saying "__aenter__() must return awaitable" is just incorrect, or CPython implementation has a bug. Actual source implies former. _ContextManagerMixin in asyncio.locks does not return awaitable at all, for example.

Anyway, I believe there are inconsistencies around here.

----------
assignee: docs at python
components: Documentation
messages: 297542
nosy: dmiyakawa, docs at python
priority: normal
severity: normal
status: open
title: Inconsistent or wrong documentation around Asynchronous Context Manager
versions: Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30831>
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