A 20 long will probably be a little too small for an adult ball python.

A 40 long or 40 gallon "breeder" tank is perfect if your going to use a glass tank.

If you don't have it already, get yourself a copy of "The Ball Python Manual" .... it's available on in book stores and on Amazon.com for under $10 and had all the information you need to taking care of your snake.

-adam