FYI Chris has started a parallel discussion on the same topic here:
https://bugs.python.org/issue32363. Chris, let's keep this discussion in one
place (now it's this list, I guess). It's hard to handle the same discussion in
two different places. Please don't split discussions like this.
I'll summarize what I said in the above referenced bpo here:
1. Task.set_result() and Task.set_exception() have never ever worked properly.
They never actually communicated the set result/exception to the underlying
coroutine. The fact that they were exposed at all was a simple oversight.
I can guess how Task.set_exception() can be implemented in theory: the
exception would be thrown into the wrapped coroutine. But I don't quite
understand how Task.set_result() can be implemented at all.
2. Task and coroutine maintain a simple relationship: Task wraps its coroutine.
The result of the coroutine is the result of the Task (not the other way
around). The Task can request its coroutine to cancel. The coroutine may
ignore that request by ignoring the asyncio.CancelledError exception.
If the latter happens, the Task cannot terminate the coroutine, this is by
design. Moreover, you can always write
while True:
try:
await asyncio.sleep(1)
except:
pass
and then nothing can terminate your coroutine. IOW, if your code chooses to
ignore CancelledError the Task can do nothing about it.
3. For proper bi-directional communication between coroutines asyncio has
queues. One can easily implement a message queue to implement injection of an
exception or result into a coroutine.
[Chris]
> I was asking if there is a way to end such a task. Is there?
No, there's no way to end tasks like that. The key question here is: is this a
theoretical problem you're concerned with? Or is this something that happens
in real-world framework/library/code that you're dealing with?
Yury
> On Feb 19, 2019, at 2:53 PM, Chris Jerdonek <chris.jerdo...@gmail.com> wrote:
>
> I have an asyncio question.
>
> In Python 3.7, is there a way to reliably end a task after having
> already tried calling cancel() on it and waiting for it to end?
>
> In Python 3.6, I did this with task.set_exception(), but in 3.7 that
> method was removed.
>
> --Chris
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