Author yselivanov
Recipients Ilya.Kulakov, asvetlov, chris.jerdonek, yselivanov
Date 2019-02-19.19:54:20
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Message-id <1550606060.54.0.648293204998.issue32363@roundup.psfhosted.org>
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>> There's no clear reason to complicate the Task<->coroutine relationship by allowing to inject arbitrary exceptions into running coroutines.

> My comment was more about CancelledError rather than arbitrary exceptions. You didn't reply to the part of my response saying that Task.cancel() doesn't let you communicate why the task was ended, [..]

I didn't reply because you didn't clearly articulate why the cancellation reason is important :)  AFAIK no other async framework allows you to do this.  So what are those real-world cases that can explain why asyncio should support this functionality?

> [..] which is something the removed API's let you do.

Unfortunately they didn't.  They never worked properly; IIRC Task.set_exception had never worked at all.  The fact they were available at all was a simple oversight.

> Task.set_exception() and Task.set_result() both give you a way to unconditionally end a task. With cancel() though, the docs say, "Task.cancel() does not guarantee that the Task will be cancelled." [1]

Yes, because Task can only signal its coroutine that it was requested to be cancelled.  The coroutine may choose to ignore that by swallowing CancelledError.

> The reason you might want to unconditionally end a task is if e.g. you already called Task.cancel() and it is still running after waiting a certain amount of time.

Well, if you want to unconditionally end tasks you shouldn't write coroutines that ignore CancelledErrors.  If you do, then you clearly *do not want* the coroutine to be cancelled, and thus there's no use case for set_exception.

Finally, asyncio has queues, which can be used to build two-way communication channels between coroutines and implement whatever advanced cancellation mechanism you want.
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Date User Action Args
2019-02-19 19:54:20yselivanovsetrecipients: + yselivanov, asvetlov, chris.jerdonek, Ilya.Kulakov
2019-02-19 19:54:20yselivanovsetmessageid: <1550606060.54.0.648293204998.issue32363@roundup.psfhosted.org>
2019-02-19 19:54:20yselivanovlinkissue32363 messages
2019-02-19 19:54:20yselivanovcreate