ABCMeta.__subclasscheck__() doesn't support duck typing.

[grahamd]

BPO	44847

^{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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assignee = None
closed_at = None
created_at = <Date 2021-08-06.06:56:59.358>
labels = ['type-bug', 'library', '3.9', '3.10']
title = "ABCMeta.__subclasscheck__() doesn't support duck typing."
updated_at = <Date 2021-08-06.06:56:59.358>
user = 'https://bugs.python.org/grahamd'

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activity = <Date 2021-08-06.06:56:59.358>
actor = 'grahamd'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-08-06.06:56:59.358>
creator = 'grahamd'
dependencies = []
files = []
hgrepos = []
issue_num = 44847
keywords = []
message_count = 1.0
messages = ['399060']
nosy_count = 1.0
nosy_names = ['grahamd']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44847'
versions = ['Python 3.9', 'Python 3.10']

[grahamd]

The Python standard library has two effective implementations of helpers for the ABCMeta class. A C implementation, and a pure Python version which is only used if the C implementation isn't available (perhaps for PyPy).

• https://github.com/python/cpython/blob/3.9/Lib/abc.py#L89
• https://github.com/python/cpython/blob/3.9/Lib/_py_abc.py
• https://github.com/python/cpython/blob/3.9/Modules/_abc.c

These two implementations behave differently.

Specifically, the ABCMeta.__subclasscheck__() implementation for the C version doesn't support duck typing for the subclass argument to issubclass() when this delegates to ABCMeta.__subclasscheck__(). The Python implementation for this has no problems though.

In the pure Python version it uses isinstance().

• https://github.com/python/cpython/blob/3.9/Lib/_py_abc.py#L110

In the C implementation it uses PyType_Check() which doesn't give the same result.

• https://github.com/python/cpython/blob/3.9/Modules/_abc.c#L610

The consequence of this is that transparent object proxies used as decorators on classes (eg., as wrapt uses) will break when the C implementation us used with an error of:

    #       def __subclasscheck__(cls, subclass):
    #           """Override for issubclass(subclass, cls)."""
    #   >       return _abc_subclasscheck(cls, subclass)
    #   E       TypeError: issubclass() arg 1 must be a class

Example of tests from wrapt and how tests using C implementation must be disabled can be found at:

• https://github.com/GrahamDumpleton/wrapt/blob/develop/tests/test_inheritance_py37.py

If instead of using PyType_Check() the C implementation used PyObject_IsInstance() at that point it is possible that wrapt may then work if the remainder of the C implementation is true to how the pure Python version works (not been able to test if that is the case or not as yet).