Mark Pilgrim's Dive Into Python 3 is a hands-on guide to Python 3 and its differences from Python 2. As in the original book, Dive Into Python, each chapter starts with a real, complete code sample, proceeds to pick it apart and explain the pieces, and then puts it all back together in a summary at the end.
This book includes:

• Example programs completely rewritten to illustrate powerful new concepts now available in Python 3: sets, iterators, generators, closures, comprehensions, and much more
• A detailed case study of porting a major library from Python 2 to Python 3
• A comprehensive appendix of all the syntactic and semantic changes in Python 3

This is the perfect resource for you if you need to port applications to Python 3, or if you like to jump into languages fast and get going right away.