A series of articles that teaches you how to make the best use of the core Python features. The Pydon'ts are available as an e-book that you can read for free below.
This article serves as a complete reference for all the non-trivial things you should know about Python functions.
Think of a number between 1 and 10. Multiply it by 9 and subtract 1. Now close your eyes. It got dark, didn't it?