KeyboardInterrupt is not catched when blocking work is done

Question

To stop the following code one will need to press Ctrl-C twice, however the second raised KeyboardInterrupt exception is not catched by the inner try-catch inside the start_consuming method.

import asyncio


async def start_consuming():
    try:
        await asyncio.Future()
    except:
        try:
            await asyncio.Future()
        except:
            pass


async def main():
    await asyncio.gather(start_consuming())


if __name__ == '__main__':
    asyncio.run(main())

Why?

Answer 1

It is not like it seems. Here is your program with some debug prints added.

import asyncio

async def start_consuming():
    try:
        await asyncio.Future()
    except BaseException as b:
        print(1, repr(b))

    try:
        await asyncio.Future()
    except BaseException as b:
        print(2, repr(b))

async def main():
    try:
        await asyncio.gather(start_consuming())
    except BaseException as b:
        print(3, repr(b))

if __name__ == '__main__':
    try:
        asyncio.run(main())
    except BaseException as b:
        print (4, repr(b))

The output slightly edited:

^C 1 CancelledError()

^C 2 CancelledError()

3 CancelledError()

4 KeyboardInterrupt()

I don't have a fully detailed explanation, but it is clear that when the ctrl-C interrupts arrive, the start_consuming coroutine is not executing, because it is not ready.

The interrupt (exception) is then caught ("catched") by the asyncio itself. Everything else is in my opinion an asyncio shutdown & cleanup in action. The ctrl-C interrupt is then raised by asyncio.run() during the exit from asyncio.

I'd like to add a note regarding the asyncio shutdown, "swallowing" the cancellation exception - which is also BaseException - is against the rules. Docs: "This exception can be caught to perform custom operations when asyncio Tasks are cancelled. In almost all situations the exception must be re-raised."