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wait_for(gather(...)) logs weird error message #73618
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tecki commented
on Feb 3, 2017 teckimannequinMannequinAuthorwhen waiting for a gather that times out, everything works as expected, yet a weird error message is logged. To give a minimal example:
import asyncio @asyncio.coroutine def main(): try: sleep = asyncio.sleep(0.2) yield from asyncio.wait_for(asyncio.gather(sleep), timeout=0.1) except asyncio.TimeoutError: print("timed out: fine") yield from asyncio.sleep(0.1)
asyncio.get_event_loop().run_until_complete(main())This outputs:
timed out: fine _GatheringFuture exception was never retrieved future: <_GatheringFuture finished exception=CancelledError()> concurrent.futures._base.CancelledErrorAs you can see, I used the pre-3.5 syntax so I could test whether it works on older systems. No, it doesn't.
I wrote a unit test for this problem, unfortunately I couldn't solve it yet.
tecki commented
on Feb 8, 2017 teckimannequinMannequinAuthorI added a solution to this problem. I just silence the bad error message by overwriting _GatheringFuture.__del__ to do nothing. This may have undesired side effects, though.
I noticed that the future defined by asyncio.gather(sleep) is in a "pending" state immediately after the asyncio.TimeoutError.
One workaround is to wait for the cancellation to finish:
@asyncio.coroutine def main(): sleep = asyncio.sleep(0.2) future = asyncio.gather(sleep) try: yield from asyncio.wait_for(future, timeout=0.1) except asyncio.TimeoutError: print(f'future: {future}') try: yield from future except asyncio.CancelledError: print(f'future: {future}') yield from asyncio.sleep(0.1)
asyncio.get_event_loop().run_until_complete(main())Outputs:
future: <_GatheringFuture pending> future: <_GatheringFuture finished exception=CancelledError()>Another option is to pass return_exceptions=True to gather(). This appears to make the log messages you were concerned about go away:
future: <_GatheringFuture pending>By the way, I see this exact issue was also raised and discussed here, with a couple responses by Guido, too:
python/asyncio#253it looks like this was fixed in python 3.9:
graingert@superjacent ~/projects python3.9 demo.py /home/graingert/projects/demo.py:4: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead def main(): timed out: fine graingert@superjacent ~/projects python3.8 demo.py demo.py:4: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead def main(): timed out: fine _GatheringFuture exception was never retrieved future: <_GatheringFuture finished exception=CancelledError()> asyncio.exceptions.CancelledErrorlooks like it was fixed by this https://github.com/python/cpython/pull/20054/files#diff-429f4ed1e0f89ea2c92e2a8e8548ea8ae1a3d528979554fbfa4c38329e951529R503
Happy to close it.
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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