Asyncio: Queue consumer gets out of while loop without break. Where exactly and how?

[deanhystad]

An exception is raised by "name = await q.get()". It is a very quiet exception, but it kicks you out of say_hello().

If I remove return_exceptions=True from this line the exception gets a lot less quiet. :

    await asyncio.gather(*workers)

Error:
Traceback (most recent call last):
  File "C:\Python\Python39\lib\asyncio\queues.py", line 166, in get
    await getter
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "myprogram", line 23, in main
    await asyncio.gather(*workers)
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "myprogram", line 25, in <module>
    asyncio.run(main())
  File "C:\Python\Python39\lib\asyncio\runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "C:\Python\Python39\lib\asyncio\base_events.py", line 642, in run_until_complete
    return future.result()
asyncio.exceptions.CancelledError

exception asyncio.CancelledError
The operation has been cancelled.

This exception can be caught to perform custom operations when asyncio Tasks are cancelled. In almost all situations the exception must be re-raised.

Here I will catch the cancelled error so I can perform some important cleanup.

import asyncio
names = ['Aris', 'Kostas', 'Nikos', 'Makis', 'Christos', 'Andreas', 'Vasilis', 'Thanasis', 'Petros', 'Pavlos']

async def say_hello(id, q):
    while True:
        try:
            name = await q.get()
            print(f'{id} {name}')
            await asyncio.sleep(0.1)
            q.task_done()
        except asyncio.CancelledError as msg:
            print(msg)
            break
    print(f"{id} cancelled")

async def main():
    q = asyncio.Queue()

    for name in names:
        q.put_nowait(name)

    workers = [asyncio.create_task(say_hello(i, q)) for i in range(4)]

    await q.join()
    for worker in workers:
        worker.cancel()

    await asyncio.gather(*workers)

asyncio.run(main())

Output:
0 Aris
1 Kostas
2 Nikos 
3 Makis 
0 Christos
2 Andreas 
1 Vasilis 
3 Thanasis
0 Petros
1 Pavlos

0 cancelled

1 cancelled

2 cancelled

3 cancelled

Wow, cancelled without even leaving behind a note. That's cold.