We lost cmp in python3

This is because of PEP207. They proposed rich comparison; They are <, >, <=, >=, ==, !=, which associates to lt, gt, le, ge, eq, and ne. cmp is no longer used in it.
SAD NEWS

Now when you want to pass a comparison to sort you might have to define your own.
The 2.7 handbook says:

cmp(x, y)

Compare the two objects x and y and return an integer according to the outcome. The return value is negative if x < y, zero if x == y and strictly positive if x > y.

So we can write:

def my_cmp(x,y):
    (a > b) - (a < b)

This is actually the official work around in the documentation.FOR REAL
It will return 1 if a > b, -1 if a < b and 0 if a == b