from asyncio import CancelledError, coroutine, ensure_future, gather, get_event_loop, sleep

tasks = []


def cancel_task(_):
    print('Cancelling the task:', tasks[0])
    print('Cancellation returned: ', tasks[0].cancel())

@coroutine
def gather_test():
    task = ensure_future(sleep(0.1))

    # cancel_task is only added as a callback here since it guarantees
    # delivery of the cancellation at the 'unfortunate' time that
    # triggers the bug. It can also occur if cancellation is triggered
    # otherwise. However, that is very hard to set up for a test case.
    task.add_done_callback(cancel_task)

    yield from gather(task)
    print('Finished gathering.')
    yield from sleep(1)
    print('Proof that this is running even though it was cancelled')

@coroutine
def main():
    try:
        tasks.append(ensure_future(gather_test()))
        yield from tasks[0]
    except CancelledError:
        print('CancelledError in main()')
    yield from sleep(2)

get_event_loop().run_until_complete(main())