Hi,

In general, registering a class with an ABC is equivalent to making it a subclass (isinstance and issubclass are patched through ABCMeta).  However, this does not work for exceptions (see example below, where exception is not caught).

This doesn't seem terribly surprising to me - I imagine that checking would slow down exception handling - but I couldn't find any documentation (and posting on c.l.p didn't turn up anything either).

So I thought I would raise it here - perhaps there is a possible fix (my obscure use case is that I have a backtracking search; backtracking occurs when a certain exception is encountered; making that exception an ABC and allowing existing exceptions to be registered with it allows the search to work with existing code without a wrapper that catches and translates exceptions that should trigger a backtrack).  Or perhaps the docs could be extended.  Or perhaps I've misunderstood something...

Cheers,
Andrew

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