Python Types and Objects
http://www.cafepy.com/article/python_types_and_objects/python_types_and_objects.html
http://www.cafepy.com/article/python_types_and_objects/python_types_and_objects.html
Subtyping Built-in Types
http://www.python.org/dev/peps/pep-0253/
Unifying types and classes in Python
http://www.python.org/download/releases/2.2.3/descrintro/