Type Checking

When checking the types of parameters to an operator method, be aware that the Python functions isinstance() and issubclass() can be fooled by objects that redefine their __class__ attribute. This is all right in Python, but it's not acceptable for something that needs to access the C attributes of an object. It's better (as well as faster) to use the PyObject_IsInstance() and PyObject_IsSubclass() C API functions. You can gain access to them with the following declarations:

cdef extern from "Python.h":
    int PyObject_IsInstance(object, object)
    int PyObject_IsSubclass(object, object)