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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