Classmethod properties are erroneously "called" in multiple modules

[rzepeckit]

BPO	44904
Nosy	@tim-one, @rhettinger, @serhiy-storchaka, @miss-islington, @AlexWaygood
PRs	bpo-44904: Fix classmethod property bug in doctest module #28838 [3.10] bpo-44904: Fix classmethod property bug in doctest module (GH-28838) #29261 [3.9] bpo-44904: Fix classmethod property bug in doctest module (GH-28838) #29262
Superseder	bpo-45356: Calling help executes @classmethod @Property decorated methods
Files	bug_report.py: Example of the issue classmethod_properties_bug.py: Short script demonstrating how a bug relating to classmethod properties causes doctest.py to crash

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-10-10.22:18:34.456>
created_at = <Date 2021-08-12.21:16:41.103>
labels = ['type-bug', '3.9', '3.10', '3.11']
title = 'Classmethod properties are erroneously "called" in multiple modules'
updated_at = <Date 2021-10-28.08:13:53.897>
user = 'https://bugs.python.org/rzepeckit'

bugs.python.org fields:

activity = <Date 2021-10-28.08:13:53.897>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2021-10-10.22:18:34.456>
closer = 'rhettinger'
components = []
creation = <Date 2021-08-12.21:16:41.103>
creator = 'rzepecki.t'
dependencies = []
files = ['50212', '50218']
hgrepos = []
issue_num = 44904
keywords = ['patch']
message_count = 6.0
messages = ['399482', '399622', '403612', '405154', '405156', '405157']
nosy_count = 6.0
nosy_names = ['tim.peters', 'rhettinger', 'serhiy.storchaka', 'miss-islington', 'AlexWaygood', 'rzepecki.t']
pr_nums = ['28838', '29261', '29262']
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '45356'
type = 'behavior'
url = 'https://bugs.python.org/issue44904'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[rzepeckit]

Subclassing an abc with an abstract class property yields to unexpected behaviour: the class property is called, and an abstract class may be erroneously considered concrete.

See https://stackoverflow.com/a/68763572/4434666 for details.

[AlexWaygood]

This same bug (where classmethod properties are accidentally called by other parts of Python) is also present for non-abstract class properties, and has the side effect of causing doctest.py to crash with a fairly incomprehensible AttributeError:

Script:

    class Untestable:
        """
        >>> Untestable.my_classmethod_property
        'Oh no!'
        """

        @classmethod
        @property
        def my_classmethod_property(cls):
            return 'Oh no!'


    if __name__ == '__main__':
        import doctest
        doctest.testmod()

Output:

    Traceback (most recent call last):
      File "C:/Users/Alex/classmethod_properties_bug.py", line 13, in <module>
        doctest.testmod()
      File "C:\Users\Alex\.conda\envs\WebGame39\lib\doctest.py", line 1955, in testmod
        for test in finder.find(m, name, globs=globs, extraglobs=extraglobs):
      File "C:\Users\Alex\.conda\envs\WebGame39\lib\doctest.py", line 939, in find
        self._find(tests, obj, name, module, source_lines, globs, {})
      File "C:\Users\Alex\.conda\envs\WebGame39\lib\doctest.py", line 1001, in _find
        self._find(tests, val, valname, module, source_lines,
      File "C:\Users\Alex\.conda\envs\WebGame39\lib\doctest.py", line 1028, in _find
        val = getattr(obj, valname).__func__
    AttributeError: 'str' object has no attribute '__func__'

[rhettinger]

Moving this discussion to bpo-45356 because some of the discussion would need to be duplicated.

[serhiy-storchaka]

New changeset b1302ab by Alex Waygood in branch 'main':
bpo-44904: Fix classmethod property bug in doctest module (GH-28838)
b1302ab

[miss-islington]

New changeset 1f45cc0 by Miss Islington (bot) in branch '3.10':
bpo-44904: Fix classmethod property bug in doctest module (GH-28838)
1f45cc0

[miss-islington]

New changeset 8365a5b by Miss Islington (bot) in branch '3.9':
bpo-44904: Fix classmethod property bug in doctest module (GH-28838)
8365a5b

[SharpEdgeMarshall]

This is not fixed, you just fixed an issue on doctest module but the issue persist if you create an abstract class property