I found some issue that I suppose is a bug. Let us have long running coroutines. We use them in gather, and one of them raises an error. Since then we cannot cancel the gather anymore, thus remaining children are not cancelable and executed until complete or raise an exception themselves. === import asyncio async def coro_with_error(): # Coro raises en error with 1 sec delay await asyncio.sleep(1) raise Exception('Error in coro') async def cancellator(coro): # We use this to cancel gather with delay 1 sec await asyncio.sleep(1) coro.cancel() async def success_long_coro(): # Long running coro, 2 sec try: await asyncio.sleep(2) print("I'm ok!") return 42 except asyncio.CancelledError: # Track that this coro is really cancelled print('I was cancelled') raise async def collector_with_error(): gather = asyncio.gather(coro_with_error(), success_long_coro()) try: await gather except Exception: print(f"WHOAGH ERROR, gather done={gather.done()}") print(f'EXC={type(gather.exception()).__name__}') # We want to cancel still running success_long_coro() gather.cancel() async def collector_with_cancel(): # Gather result from success_long_coro() gather = asyncio.gather(success_long_coro()) # schedule cancel in 1 sec asyncio.create_task(cancellator(gather)) try: await gather except Exception: print(f"WHOAGH ERROR, gather done={gather.done()}") print(f'EXC={type(gather.exception()).__name__}') # We want to cancel still running success_long_coro() gather.cancel() return # First case, cancel gather when children are running print('First case') loop = asyncio.get_event_loop() loop.create_task(collector_with_cancel()) # Ensure test coros we fully run loop.run_until_complete(asyncio.sleep(3)) print('Done') # Second case, cancel gather when child raise error print('Second case') loop = asyncio.get_event_loop() loop.create_task(collector_with_error()) # Ensure test coros we fully run loop.run_until_complete(asyncio.sleep(3)) print('Done') === Actual output: First case I was cancelled WHOAGH ERROR, gather done=True EXC=CancelledError Done Second case WHOAGH ERROR, gather done=True EXC=Exception I'm ok! Done Expected output: First case I was cancelled WHOAGH ERROR, gather done=True EXC=CancelledError Done Second case I was cancelled WHOAGH ERROR, gather done=True EXC=Exception Done Documentations says: > If gather() is cancelled, all submitted awaitables (that have not completed yet) are also cancelled. But it mentions no cases on child coros' exceptions. From doc: > If return_exceptions is False (default), the first raised exception is immediately propagated to the task that awaits on gather(). Other awaitables in the aws sequence won’t be cancelled and will continue to run. Which is true, exception is propagated, the gather has an exception set, marked as done() so its children are not cancelled. I believe asyncio should allow cancellation in that case.

Hi Dimitri, You are right, gather.cancel() doesn't work once it has propagated an exception. This happens because after propagating the exception to the caller, gather is marked as done. And cancel doesn't work after a Future object has been marked done. You can test the same by printing the return value of gather.cancel(). It will be False I also believe that the documentation of gather should explicitly mention this. But, depending on the fact, whether this is an expected behaviour, current code base might also need changes. Therefore I have created two patches, one updating the current documentation according to the current functionality, and other changing the codebase which supports cancelling even after raising exceptions. I will try to contact one of the core developers on deciding which one is the way to go.

The behavior change is not backward compatible, we cannot apply it. Also, please use github pull requests for proposing a patch: https://devguide.python.org/pullrequest/

Hi Andrew, Thanks for replying! I understand that the behavior change is not backward compatible. But the current behavior is a bit ambiguous ad mentioned by Dimitri, therefore I have updated the documentation and opened a Pull request. I would be very glad if you could review it. Thanks! Also, would adding an optional argument like force_cancel=False to ``gather.cancel``, which won't return even if gather is done be backward compatible. Should I open a PR with this change also, so that you can better review it and see if it looks good.

 New changeset d42528a3a2c7d79fd2e6c9f2a02f3ce12d44c8cc by Vinay Sharma in branch 'master': bpo-37703: improve asyncio.gather documentation regarding cancellation (GH-15312) https://github.com/python/cpython/commit/d42528a3a2c7d79fd2e6c9f2a02f3ce12d44c8cc 

 New changeset 58f59a962180123a6d29ece512d198b365726b33 by Miss Islington (bot) in branch '3.8': bpo-37703: improve asyncio.gather documentation regarding cancellation (GH-15312) https://github.com/python/cpython/commit/58f59a962180123a6d29ece512d198b365726b33 

 New changeset 46634b7aa82f014cd0039afb7f0ed860605beb9d by Miss Islington (bot) in branch '3.9': bpo-37703: improve asyncio.gather documentation regarding cancellation (GH-15312) https://github.com/python/cpython/commit/46634b7aa82f014cd0039afb7f0ed860605beb9d