[Async-sig] subclassing CancelledError

[Guido van Rossum]

Sounds like an implementation issue. The Future class has a boolean flag
indicating whether it's been cancelled, and everything then raises
CancelledError when that flag is set. I suppose we could replace that flag
with an instance of CancelledError, and when we *catch* CancelledError we
set it to that. But it seems messy and I'm not sure that you should attempt
this. IMO you should define an exception that does *not* derive from
CancelledError and raise that -- it will properly be transmitted. What's
your reason for not doing that? IOW why are you trying to pump more than a
bit through the narrow "cancelled" channel?

On Sun, Dec 3, 2017 at 2:11 AM, Andrew Svetlov <andrew.svetlov at gmail.com>
wrote:

> 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
>> _______________________________________________
>> Async-sig mailing list
>> Async-sig at python.org
>> https://mail.python.org/mailman/listinfo/async-sig
>> Code of Conduct: https://www.python.org/psf/codeofconduct/
>>
> --
> Thanks,
> Andrew Svetlov
>
> _______________________________________________
> Async-sig mailing list
> Async-sig at python.org
> https://mail.python.org/mailman/listinfo/async-sig
> Code of Conduct: https://www.python.org/psf/codeofconduct/
>
>


-- 
--Guido van Rossum (python.org/~guido)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/async-sig/attachments/20171203/157464e3/attachment.html>