ExtensionClasses is a lightweight mechanism has been developed for
making Python extension types more class-like.  Classes can be developed
in an extension language, such as C or C++, and these classes can be
treated like other python classes.  They can be sub-classed in python,
they provide access to method documentation strings, and they can be
used to directly create new instances.

Extension classes provide additional extensions to class and instance
semantics, including A protocol for accessing subobjects "in the context
of" their containers. This is used to implement custom method types and
environmental acquisition*. A protocol for overriding method call
semantics. This is used to implement "synchonized" classes and could be
used to implement argument type checking. A protocol for class
initialization that supports execution of a special __class_init__
method after a class has been initialized.

(* see http://www.digicool.com/releases/ExtensionClass/Acquisition.html)