eryksun added the comment:
Given super(cls, obj), cls needs to be somewhere in type(obj).__mro__. Thus the
implementation checks PyType_IsSubtype instead of the more generic
PyObject_IsSubclass.
In this case int's MRO is unrelated to numbers.Number:
>>> print(*int.__mro__, sep='\n')
<class 'int'>
<class 'object'>
It gets registered as a subclass via numbers.Integral.register(int).
>>> print(*numbers.Integral._abc_registry)
<class 'int'>
issubclass calls PyObject_IsSubclass, which uses the __subclasscheck__ API. In
this case ABCMeta.__subclasscheck__ recursively checks the registry and caches
the result to speed up future checks.
>>> numbers.Number.__subclasscheck__(int)
True
>>> print(*numbers.Number._abc_cache)
<class 'int'>
----------
nosy: +eryksun
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue20503>
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