The Python Tutorial, by Guido van Rossum
This is the standard tutorial reference by the inventor of Python. Everyone should have a bookmark for it in their browser for reference.
The official Python IDLE documentation, including keyboard shortcuts, debugging, etc.
Local/Global Variables (Scoping)
If you are interested in learning more about scoping, try reading chapter 3 of the 6.01SC course notes (start at section 3.2.2, page 42).
Also, here's a good blog post: Notes on Python variable scope