PyObject_IsInstance() is used incorrectly for testing if Python object is an instance of specified builtin type before direct access to internals of object. This is not correct, because PyObject_IsInstance() checks the __class__ attribute that can be modified and even can be dynamic property. Correct way is to check static type. Proposed patch replaces PyObject_IsInstance() with PyObject_TypeCheck() if this is appropriate.

See also similar issues issue24102 and issue24091.