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Occasional incorrect chaining of CancelledError when calling 'cancel' on result of 'asyncio.gather' #97907

Description

@vladima
import asyncio

async def f1():
    await asyncio.sleep(1)

async def f2():
    return 42

async def main():
    # case 1
    gfut = asyncio.gather(f1(), f2(), return_exceptions=True)
    # case 2
    #gfut = asyncio.gather(f2(), f1(), return_exceptions=True)
    await asyncio.sleep(0.1)
    gfut.cancel("my message")
    await gfut

asyncio.run(main())

# case 1

#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 14, in main
#    print(await gfut)
#asyncio.exceptions.CancelledError
#
#During handling of the above exception, another exception occurred:
#
#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 16, in <module>
#    asyncio.run(main())
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/runners.py", line 44, in run
#    return loop.run_until_complete(main)
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/base_events.py", line 646, in run_until_complete
#    return future.result()
#asyncio.exceptions.CancelledError

# case 2
#Traceback (most recent call last):
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/tasks.py", line 605, in sleep
#    return await future
#asyncio.exceptions.CancelledError: my message

#During handling of the above exception, another exception occurred:

#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 14, in main
#    print(await gfut)
#asyncio.exceptions.CancelledError
#
#During handling of the above exception, another exception occurred:
#
#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 16, in <module>
#    asyncio.run(main())
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/runners.py", line 44, in run
#    return loop.run_until_complete(main)
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/base_events.py", line 646, in run_until_complete
#    return future.result()
#asyncio.exceptions.CancelledError

Currently traceback and description parts of CancelledError depend upon the order of arguments that were passed into asyncio.gather. The reason I believe is this: here fut would be bound to the last item in children which might or might not correspond to the cancelled task and instead it should probably be one of tasks that were actually cancelled (which is still not ideal since there might be many of them but I guess with gather it is as good as it can be).

Linked PRs

Activity

  1. added
    type-bugAn unexpected behavior, bug, or error
    on Oct 5, 2022
  2. akulakov commented on Oct 15, 2022

    @akulakov
    Contributor

    Note that msg argument to cancel() is deprecated in 3.11 and will be removed in the future, so once that happens it will be just that traceback is different - not sure if that's an issue that needs a fix.

  3. gvanrossum commented on Oct 15, 2022

    @gvanrossum
    Member

    Actually on the final 3.11 release cancel(msg) will be undeprecated — we had a last minute change of heart. So if there’s an issue it may have to be addressed — in 3.12, probably.

  4. akulakov commented on Oct 15, 2022

    @akulakov
    Contributor

    @gvanrossum in that case should I remove the deprecation warning from latest dev branch, or that's already planned to be done by someone?

  5. gvanrossum commented on Oct 15, 2022

    @gvanrossum
    Member

    We already removed it.
    #98006

  6. gvanrossum commented on Oct 22, 2022

    @gvanrossum
    Member

    On the original issue, I suppose we could do better -- a PR would be appreciated.

  7. added a commit that references this issue on May 1, 2025
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