Python changed it's sorting methods to accept a key function. Those functions take a value and return a key which is used to sort the arrays.
Old comparison functions used to take two values and return -1, 0 or +1 if the first argument is small, equal or greater than the second argument respectively. This is incompatible to the new key-function.
functools.cmp_to_key comes in:
>>> import functools >>> import locale >>> sorted(["A", "S", "F", "D"], key=functools.cmp_to_key(locale.strcoll)) ['A', 'D', 'F', 'S']
Example taken and adapted from the Python Standard Library Documentation.