How to clear Python shell

Unfortunately, I don't think there is a way to clear the screen in IDLE. The best you could do is to scroll the screen down lots of lines, eg:
print "\n" * 100
Though you could put this in a function:
def cls(): print "\n" * 100
And then call it when needed as cls().

However, if you are using the command line, you can use the following to clear the command line.
import os
os.system("cls")