"Dive Into Python" is a free Python book for experienced programmers. Chapter 5, "Unit Testing", is now available: http://diveintopython.org/roman_divein.html
Chapter 5 teaches unit testing with PyUnit (now known as the "unittest" module, standard in Python 2.1). It goes step by step through the full lifecycle of a module:
Each step includes full Python source code for both the test cases and the module being tested, so you can see how the code evolves from a set of function stubs that fail all test cases to a complete, working, well-tuned module.
You're smart; why haven't you learned Python yet? http://diveintopython.org/