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.

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__'

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

New changeset b1302abcc8a4be5f39b4d60a1ce28032b77655b3 by Alex Waygood in branch 'main':
bpo-44904: Fix classmethod property bug in doctest module (GH-28838)
https://github.com/python/cpython/commit/b1302abcc8a4be5f39b4d60a1ce28032b77655b3

New changeset 1f45cc0dfa9a8febfc256411c803b4536719db97 by Miss Islington (bot) in branch '3.10':
bpo-44904: Fix classmethod property bug in doctest module (GH-28838)
https://github.com/python/cpython/commit/1f45cc0dfa9a8febfc256411c803b4536719db97

New changeset 8365a5b5abe51cbe4151d89a5d0a993273320067 by Miss Islington (bot) in branch '3.9':
bpo-44904: Fix classmethod property bug in doctest module (GH-28838)
https://github.com/python/cpython/commit/8365a5b5abe51cbe4151d89a5d0a993273320067