How to use cmp() in Python 3?

As mentioned in the comments, cmp doesn’t exist in Python 3. If you really want it, you could define it yourself:

def cmp(a, b):
    return (a > b) - (a < b) 

which is taken from the original What’s New In Python 3.0. It’s pretty rare — though not unheard of — that it’s really needed, though, so you might want to think about whether it’s actually the best way to do whatever it is you’re up to.