[Async-sig] subclassing CancelledError

[Andrew Svetlov]

IIRC at very early stages Guido van Rossum decided to *freeze*
`CancelledError`: user code should not derive from the exception. Like you
never derive from StopIteration.

On Sun, Dec 3, 2017 at 8:00 AM Chris Jerdonek <chris.jerdonek at gmail.com>
wrote:

> Hi, I want to ask how people feel about the following.
>
> If you raise a subclass of CancelledError from within a task and then
> call task.result(), CancelledError is raised rather than the subclass.
>
> Here is some code to illustrate:
>
>     class MyCancelledError(CancelledError): pass
>
>     async def raise_my_cancel():
>         raise MyCancelledError()
>
>     task = asyncio.ensure_future(raise_my_cancel())
>     try:
>         await task
>     except Exception:
>         pass
>     assert task.cancelled()
>     # Raises CancelledError and not MyCancelledError.
>     task.result()
>
> Does this seem right to people? Is there a justification for this?
>
> If it would help for the discussion, I could provide a use case.
>
> Thanks a lot,
> --Chris
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-- 
Thanks,
Andrew Svetlov
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