Basic Python Ciphers

I've started reading the book "Hacking Secret Ciphers with Python" just for some fun inspiration, and the introduction is about Caesar Cipher's caught my attention. Basically, the old school 'encryption' you'd do in elementary school where A = 1, B = 2, C = 3, etc... but then you could shift it so that A = 5, B = 6, C= 7, etc... Here's the basic code I've implemented in Python so far. Haven't touched Python in at least a year, except to rewrite a basic Fibonacci Sequence formula for laughs occasionally (set it to go to 10,000 and watch the numbers print out looking like the screens from The Matrix...), so I'm quite chuffed with the results. Enjoy!