New submission from Graham Dumpleton <graham.dumple...@gmail.com>:
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). ---------- components: Library (Lib) messages: 399060 nosy: grahamd priority: normal severity: normal status: open title: ABCMeta.__subclasscheck__() doesn't support duck typing. type: behavior versions: Python 3.10, Python 3.9 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue44847> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
