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Exception handling matches subtypes, not subclasses
# Example
 from abc import ABC
 
 class MyException(Exception, ABC):
 pass
 
 class OtherException(Exception):
 pass
 
 MyException.register(OtherException)
 
 try:
 raise OtherException
 except MyException:
 print("Correct: Caught MyException")
 except Exception:
 print("Wrong: Caught something else")
 
 # "Wrong: Caught something else"
# Background and evidence of bug-ness
Issue 2534 [1] (10 years ago!) introduced the behavior, but only in the Python 3 patch [2]. During code review, the correct function call was used [3], but the function's name got switched in the final python3 patch without any comment.
The current Python 2 code uses `PyObject_IsSubclass`, and produces the correct behavior in the example above (using `__metaclass__ = ABCMeta`, of course). This leads me to strongly suspect that this is a bug, not a feature. The note below regarding unittest for further evidence that this code has eight legs.
Given the ancient nature of this bug, it affects all versions of python3.
[1] https://bugs.python.org/issue2534
[2] https://bugs.python.org/file11257/isinstance3k-2.patch
[3] https://codereview.appspot.com/483/diff/1/21#newcode114
[4] https://github.com/python/cpython/blob/2.7/Python/errors.c#L119
# Solution
Coming very soon in a PR on Github, but in short, we do the following:
 1. Switch `PyType_IsSubtype` to `PyObject_IsSubclass`.
 2. Revert the changes made to remove "dead code" in https://bugs.python.org/issue31091. The code was dead because the wrong function was used—the PR left only the bug.
 3. Add tests. Note that `unittest`’s `self.assertRaises` function uses `issubclass` and does not alert to this bug. (Different symptom, same cause.)
# Note
This bug has nothing to do with the `abc` package, beyond being a simple way to generate the error.
-Mike
Gitub: mbmccoy

Adding Serhiy because this relates to issue31091, and specifically claims that there _was_ a bug in the old code (msg299585).

Isn't this a duplicate of issue12029?

Serhiy, it sure is. I'll note that the issue is open. Moreover, reading through the history, Guido says it's a bug (msg160418), and there was agreement that it should be fixed. It's unclear why it was dropped.
Can you help me get this in? I'm not sure the correct protocol, since this is my first contrib.
Note that if this is not a bug, then there are probably bugs anywhere issubclass(...) is used to emulate exception handling. For example, unittest's self.assertRaises is inconsistent with the current behavior:
 from abc import ABC
 import unittest
 
 class TestExceptionABC(unittest.TestCase):
 
 def test_exception(self):
 class A(Exception, ABC):
 pass
 class B(Exception):
 pass
 A.register(B)
 with self.assertRaises(A, msg="Wrong exception raised"):
 raise B
Test passes in python3, but not in python2.

Let to keep the discussion in a single place -- in issue12029.