[New-bugs-announce] [issue32703] 'async with' somehow suppresses unawaited coroutine warnings

[Nathaniel Smith]

New submission from Nathaniel Smith <njs at pobox.com>:

Example (minimal version of https://github.com/python-trio/trio/issues/425):

-----

async def open_file():
    pass

async def main():
    async with open_file():  # Should be 'async with await open_file()'
        pass

coro = main()
coro.send(None)

-----

Here we accidentally left out an 'await' on the call to 'open_file', so the 'async with' tries to look up 'CoroutineType.__aexit__', which obviously doesn't exist, and the program crashes with an AttributeError("__aexit__"). Yet weirdly, this doesn't trigger a warning about 'open_file' being unawaited. It should!

Yury's theory: maybe BEFORE_ASYNC_WITH's error-handling path is forgetting to DECREF the object.

----------
components: asyncio
messages: 311052
nosy: asvetlov, giampaolo.rodola, njs, yselivanov
priority: normal
severity: normal
status: open
title: 'async with' somehow suppresses unawaited coroutine warnings
versions: Python 3.5, Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32703>
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