Mark Pilgrim's Dive Into Python is now available in book form . Reading it in print reminded me how good it was, and it has grown a few chapters since I first read it online.
If an aversion to reading online has kept you back, go buy it now. It is a very impressive piece of work - it manages to explain the Python language, and, more importantly the idioms and customs of Python programming, through a series of well-chosen and interesting examples. The chapter on test-driven development shows why this makes sense, how Python supports it, and how it leads to better code.
Mark writes as he does - look at his excellent Universal Feed Parser, with its thousands of test cases for a concrete example of the power of test-driven development.
Python is my favorite programming language. Mark does a great job of explaining why. Try it, you'll like it.