Quoted:Python 2.2 introduces the first phase of "type/class unification". This is a series of changes to Python intended to remove most of the differences between built-in types and user-defined classes. Perhaps the most obvious one is the restriction against using built-in types (such as the type of lists and dictionaries) as a base class in a class statement. Andrew Kuchling's Python Warts paper devotes one of its longest sections to this language deficiency. Incidentally, Andrew's paper What's New in Python 2.2 gives a broad overview of what else is new in Python 2.2.