Sort a list to form the largest possible number

Question

I am trying to write a function that given a list of non negative integers, arranges them such that they form the largest possible number.

For example, given

Answer 1

import functools

def cmpr(x, y):
    xy = str(x) + str(y)
    yx = str(y) + str(x)
    return -1 if (xy > yx) else 1

a = [50, 2, 1, 9]
a.sort(key=functools.cmp_to_key(cmpr))