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PyErr_ExceptionMatches must not fail, according to the docs. So an
error code from PyObject_IsSubclass() cannot be returned.
The attached patch calls PyErr_WriteUnraisable in this case, and returns
an arbitrary value (0 was chosen in the patch).
See also issue #2534.

The tests pass and prints the ignores. But I still see an issue:
import sys
def g():
 try:
 return g()
 except:
 return sys.exc_info()
>>> g()
(<type 'exceptions.RuntimeError'>, 'maximum recursion depth exceeded
while calling a Python object', <traceback object at 0xb7d3d75c>)
>>> import sys
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
RuntimeError: maximum recursion depth exceeded
Then, I can raise that RuntimeError and clear things:
>>> sys.excepthook(*g())
Traceback (most recent call last):
 File "<stdin>", line 3, in g
RuntimeError: maximum recursion depth exceeded while calling a Python object
>>> import sys
>>> 
The attached patch adds a test for this condition, I hope someone more
test-savvy than I can review it.

Daniel, IMO you've found a separate bug. I added it as issue #2548.

This is a bug, not a new feature so it's not release critical for the
first alphas.

FWIW some comments by Amaury are here:
http://codereview.appspot.com/483 

Ping? Anyone cares about this?

I am working on Thomas' patch, plus tests.

Committed r65320 in trunk.
I'll close the issue after it is merged into py3k.

I just merged it.