python - How to cancel Task execution even if it ignored CancelledError?

Question

This is example of cancelling task:

import asyncio


async def some_func():
    await asyncio.sleep(2)
    print('Haha! Task keeps running!')
    await asyncio.sleep(2)

async def cancel(task):
    await asyncio.sleep(1)
    task.cancel()

async def main():
    func_task = asyncio.ensure_future(some_func())
    cancel_task = asyncio.ensure_future(cancel(func_task))
    try:
        await func_task
    except asyncio.CancelledError:
        print('Task cancelled as expected')

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

# Task cancelled as expected
# [Finished in 1.2s]

It works ok, task was cancelled. If CancelledError caught inside some_func task wouldn't be cancelled:

async def some_func():
    try:
        await asyncio.sleep(2)
    except:
        pass
    print('Haha! Task keeps running!')
    await asyncio.sleep(2)

# Haha! Task keeps running!
# [Finished in 3.2s]

It can be easy to forgot I shouldn't suppress exceptions anywhere inside async code (or some_func can be third party code, for example), but task should be cancelled. Is there anyway I can do that? Or ignored CancelledError means task can't be cancelled at all?

Answer 1

You cannot cancel task that suppresses CancelledError. This is similar to impossibility to close generator which ignores GeneratorExit.

This is intentional behavior. Task may want to do some extra work (e.g. resource cleanup) on cancelling, thus catching CancelledError may be good idea but suppressing usually is sign of programming error.

Python usually allows you to shoot own feet if you have uncompromising intention to do this.

Catching all exceptions even forbids closing python process by pressing <Ctrl+C> because it's translated into KeyboardInterrupt internally.