[New-bugs-announce] [issue40894] asyncio.gather() cancelled() always False

[Timm Wagener]

New submission from Timm Wagener <wagenertimm at gmail.com>:

It seems like the future subclass returned by asyncio.gather() (_GatheringFuture) can never return True for future.cancelled() even after it's cancel() has been invoked successfully (returning True) and an await on it actually raised a CancelledError. This is in contrast to the behavior of normal Futures and it seems generally to be classified as a minor bug by developers.

* Stackoverflow Post: https://stackoverflow.com/questions/61942306/asyncio-gather-task-cancelled-is-false-after-task-cancel
* Github snippet: https://gist.github.com/timmwagener/dfed038dc2081c8b5a770e175ba3756b

I have created a fix and will create a PR. It seemed rather easy to fix and the asyncio test suite still succeeds. So maybe this is a minor bug, whose fix has no backward-compatibility consequences. However, my understanding is that asyncio.gather() is scheduled for deprecation, so maybe changes in this area are on halt anyways!?

----

# excerpt from snippet
async def main():
    """Cancel a gather() future and child and return it."""

    task_child = ensure_future(sleep(1.0))
    future_gather = gather(task_child)

    future_gather.cancel()
    try:
        await future_gather
    except CancelledError:
        pass

    return future_gather, task_child


# run
future_gather, task_child = run(main())

# log gather state
logger.info(future_gather.cancelled())  # False / UNEXPECTED / ASSUMED BUG
logger.info(future_gather.done())  # True
logger.info(future_gather.exception())  # CancelledError
logger.info(future_gather._state)  # FINISHED

# log child state
logger.info(task_child.cancelled())  # True
logger.info(task_child.done())  # True
# logger.info(task_child.exception())  Raises because _state is CANCELLED
logger.info(task_child._state)  # CANCELLED

----------
components: asyncio
messages: 370855
nosy: asvetlov, timmwagener, yselivanov
priority: normal
severity: normal
status: open
title: asyncio.gather() cancelled() always False
type: enhancement
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40894>
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