[descrintro] Unifying types and classes in Python 2.2. Guido van Rossum.

[pep-252] Making Types Look More Like Classes. Guido van Rossum.

[pep-253] Subtyping Built-in Types. Guido van Rossum.

[descriptors-howto] How-To Guide for Descriptors. Raymond Hettinger.

[mro-algorithm] The Python 2.3 Method Resolution Order. Michele Simionato.