[issue12029] Allow catching virtual subclasses in except clauses
Michael McCoy added the comment: Checking my comment history here, a past me was terribly bad at linking the correct PR on github.This is the correct link: https://github.com/python/cpython/pull/6461 On Mon, Feb 7, 2022 at 10:12 AM Guido van Rossum wrote: > > Guido van Rossum added the comment: > > Fixing the version field. Since it's a feature, this could not go into any > version before 3.11. > > Maybe Irit can look through the discussion and patch and see if there's > value to doing this? (Feel free to decline!) > > -- > nosy: +iritkatriel > versions: +Python 3.11 -Python 3.4, Python 3.5, Python 3.6, Python 3.7, > Python 3.8, Python 3.9 > > ___ > Python tracker > <https://bugs.python.org/issue12029> > ___ > -- ___ Python tracker <https://bugs.python.org/issue12029> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue33271] Exception handling matches subtypes, not subclasses
New submission from Michael McCoy :
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
--
components: Interpreter Core
messages: 315241
nosy: Michael McCoy
priority: normal
severity: normal
status: open
title: Exception handling matches subtypes, not subclasses
versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8
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<https://bugs.python.org/issue33271>
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[issue33271] Exception handling matches subclasses, not subtypes
Change by Michael McCoy : -- title: Exception handling matches subtypes, not subclasses -> Exception handling matches subclasses, not subtypes ___ Python tracker <https://bugs.python.org/issue33271> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue33271] Exception handling should match subclasses, not subtypes
Change by Michael McCoy : -- title: Exception handling matches subclasses, not subtypes -> Exception handling should match subclasses, not subtypes ___ Python tracker <https://bugs.python.org/issue33271> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue33271] Exception handling should match subclasses, not subtypes
Michael McCoy added the comment: Adding Serhiy because this relates to issue31091, and specifically claims that there _was_ a bug in the old code (msg299585). -- nosy: +serhiy.storchaka ___ Python tracker <https://bugs.python.org/issue33271> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue33271] Exception handling should match subclasses, not subtypes
Michael McCoy added the comment: 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. -- ___ Python tracker <https://bugs.python.org/issue33271> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue12029] Allow catching virtual subclasses in except clauses
Change by Michael McCoy : -- pull_requests: +6160 ___ Python tracker <https://bugs.python.org/issue12029> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue12029] Allow catching virtual subclasses in except clauses
Michael McCoy added the comment: Amalgamating the patch history here, I've updated the tests on Github (PR6160) to include tests for both the recursive case and ensure the correct error is propagated up if an exception occurs during the subclass check. I've also added a check to ensure that unittest's assertRaises behaves as expected. (The test currently passes on master, which is a bug if this doesn't get merged.) Finally, the PR updates the documentation for try/except. -- nosy: +Michael McCoy ___ Python tracker <https://bugs.python.org/issue12029> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue12029] Allow catching virtual subclasses in except clauses
Michael McCoy added the comment: Sorry, my last message referred to Github PR6460 / pull_request6160. -- versions: +Python 3.4, Python 3.6, Python 3.7, Python 3.8 ___ Python tracker <https://bugs.python.org/issue12029> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
