Why does this asyncio.Task never finish cancelling?

Question

If I run this on the python3 interpreter:

import asyncio

@asyncio.coroutine
def wait(n):
    asyncio.sleep(n)

loop = asyncio.get_event_loop()
fut = asyncio.async(wait(10))
fut.add_done_callback(lambda x: print('Done'))

asyncio.Task.all_tasks()

I get the following result:

{<Task pending coro=<coro() running at /usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/asyncio/coroutines.py:139> cb=[<lambda>() at <ipython-input-5-c72c2da2ffa4>:1]>}

Now if I run fut.cancel() I get True returned. But typing fut returns a representation of the task stating it is cancelling:

<Task cancelling coro=<coro() running at /usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/asyncio/coroutines.py:139> cb=[<lambda>() at <ipython-input-5-c72c2da2ffa4>:1]>

And the task never actually cancels (fut.cancelled() never returns True)

Why won't it cancel?

Answer 1

Calling task.cancel() only schedules the task to be cancelled on the next run of the event loop; it doesn't immediately cancel the task, or even guarantee that the task will be actually be cancelled when the event loop runs its next iteration. This is all described in the documentation:

cancel()

Request that this task cancel itself.

This arranges for a CancelledError to be thrown into the wrapped coroutine on the next cycle through the event loop. The coroutine then has a chance to clean up or even deny the request using try/except/finally.

Unlike Future.cancel(), this does not guarantee that the task will be cancelled: the exception might be caught and acted upon, delaying cancellation of the task or preventing cancellation completely. The task may also return a value or raise a different exception.

Immediately after this method is called, cancelled() will not return True (unless the task was already cancelled). A task will be marked as cancelled when the wrapped coroutine terminates with a CancelledError exception (even if cancel() was not called).

In your case, you're never actually starting the event loop, so the task never gets cancelled. You would need to call loop.run_until_complete(fut) (or loop.run_forever(), though that's not really the best choice for this particular case) for the task to actually end up getting cancelled.

Also, for what it's worth, it's usually easier to test asyncio code using actual scripts, rather than the interpreter, since it tends to get tedious to have to constantly rewrite coroutines and start/stop the event loop.