In https://bugs.python.org/issue32751 asyncio.wait_for behaviour was changed that when we use timeout=... and the timeout expires, it waits until task is canceled. However, in some cases inner task can trigger exception while it handles cancellation. Check the following code:


import asyncio


async def ignore_cancel_and_raise():
    try:
        await asyncio.sleep(20)
    except asyncio.CancelledError:
        raise Exception('Cancellation failed')


async def main():
    try:
        await asyncio.wait_for(ignore_cancel_and_raise(), timeout=1)
    except asyncio.TimeoutError:
        print('Timeout')

asyncio.run(main())


It will print "Timeout" and log a warning that "Task exception was never retrieved".

I think that in case inner task cancelation fails with some error, asyncio.wait_for should reraise it instead of silently losing it.

Also adding Nathaniel since he's the one that filed #32751. Nathaniel, do you agree that if an exception occurs while waiting for the cancellation, the exception should be what's raised instead of TimeoutError?

makes sense to me

On Tue, May 12, 2020 at 10:14 PM Chris Jerdonek <report@bugs.python.org> wrote:
>
>
> Chris Jerdonek <chris.jerdonek@gmail.com> added the comment:
>
> Also adding Nathaniel since he's the one that filed #32751. Nathaniel, do you agree that if an exception occurs while waiting for the cancellation, the exception should be what's raised instead of TimeoutError?
>
> ----------
> nosy: +chris.jerdonek, njs
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue40607>
> _______________________________________

> I think that in case inner task cancelation fails with some error, asyncio.wait_for should reraise it instead of silently losing it.

+1.

New changeset 382a5635bd10c237c3e23e346b21cde27e48d7fa by romasku in branch 'master':
bpo-40607: Reraise exception during task cancelation in asyncio.wait_for() (GH-20054)
https://github.com/python/cpython/commit/382a5635bd10c237c3e23e346b21cde27e48d7fa

Thank you again, Roman and all.