ABC-registered Exceptions are not caught as subclasses

[Chris Rebert]

On Sat, May 7, 2011 at 12:03 PM, andrew cooke <andrew at acooke.org> wrote:
>
> This isn't hugely surprising, but doesn't seem to be documented.  Is it a bug, or worth raising as one, or have I misunderstood?
>
>
> Python 3.2 (r32:88445, Feb 27 2011, 13:00:05)
> [GCC 4.5.0 20100604 [gcc-4_5-branch revision 160292]] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> from abc import ABCMeta
>>>> class RootException(Exception,metaclass=ABCMeta): pass
> ...
>>>> class MyException(Exception): pass
> ...
>>>> RootException.register(MyException)
>>>> try:
> ...     raise MyException
> ... except RootException:
> ...     print('caught')
> ...
> Traceback (most recent call last):
>  File "<stdin>", line 2, in <module>
> __main__.MyException
>
> If you assume that the ABC "register" class should work likeinheritance (as it does with issubclass and isinstance then you would, I think, have expected the exception above to have been caught.

Seems worth filing a bug IMO; it probably deserves clarification in
the docs if nothing else, though this behavior isn't inconsistent with
them as they're currently written (due to the vagaries of natural
language).

Cheers,
Chris