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asyncio.wait_for: process future result produced during cancelation #84849
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romasku commented
on May 18, 2020 romaskumannequinMannequinAuthorIn https://bugs.python.org/issue40607 asyncio.wait_for behavior was changed so it propagates exceptions that happened during cancellation. But it still raises
TimeoutErrorif cancelation ends with some value being returned. In the following example value42is lost:import asyncio async def return_42_on_cancel(): try: await asyncio.sleep(20) except asyncio.CancelledError: return 42 # `return` is useless in this block. async def main(): try: await asyncio.wait_for(return_42_on_cancel(), timeout=1) except asyncio.TimeoutError: print('Timeout') asyncio.run(main())
I think it's better to either:
- Return that value from
asyncio.wait_for.
The motivation here is that if the task returns something, we shouldn't conceal it. I also searched through GitHub and found some places where others catch
CancelledErrorand return value (https://github.com/grpc/grpc/blob/44fb37c99f2853cc23f04fba15468980d9e28e41/src/python/grpcio/grpc/experimental/aio/_interceptor.py#L328).It can also be used with some coroutines developed to be wrapped with
wait_for, for example suppose the following equation solving function:async def solve_iteratively(initial_x, next_approximation): result = initial_x try: while True: result = next_approximation(result) await asyncio.sleep(0) except asyncio.CancelledError: return result
It allows us to control its execution time using asyncio.wait_for.
- Add some warning about the value is thrown away (in debug mode) and document it somewhere.
===
I am a newbie here, so sorry if it is wrong to create such "proposal" issues.
- Return that value from
- Add some warning about the value is thrown away (in debug mode) and document it somewhere.
The documentation update is definitely something that needs to be done in 3.9. Want to submit a PR?
We can also issue a warning in asyncio debug mode.
I'm really not sure about "1) Return that value from
asyncio.wait_for.".I am a newbie here, so sorry if it is wrong to create such "proposal" issues.
The issue is clear, thanks for submitting it! Keep up the great work.
Regarding the documentation, I'm not sure we _need_ to say what happens in this edge case for 3.9. It was already unspecified before 3.9, so we're not any worse off. (The change in bpo-40607 was, however, documented.)
I'd rather come to agreement on (1) first. Because if we document it now, then it could be interpreted as defined behavior and so harder to change later due to backwards compat guarantees.
Duplicate of #86296
- marked this as a duplicate of asyncio.wait_for: process future result produced during cancelation #84849
on Oct 22, 2022 - marked this as a duplicate of bpo-42130: Fix swallowing of cancellation by wait_for #26097
on Oct 22, 2022 Duplicate of #86296
- marked this as a duplicate of AsyncIO's wait_for can hide cancellation in a rare race condition #86296
on Oct 22, 2022 (And FWIW I think there we're planning to propagate CancelledError in that case.)
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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