functools.cmp_to_key()

functools.cmp_to_key(func)

Transform an old-style comparison function to a key function. Used with tools that accept key functions (such as sorted(), min(), max(), heapq.nlargest(), heapq.nsmallest(), itertools.groupby()). This function is primarily used as a transition tool for programs being converted from Python 2 which supported the use of comparison functions.

A comparison function is any callable that accept two arguments, compares them, and returns a negative number for less-than, zero for equality, or a positive number for greater-than. A key function is a callable that accepts one argument and returns another value to be used as the sort key.

Example:

sorted(iterable, key=cmp_to_key(locale.strcoll))  # locale-aware sort order

For sorting examples and a brief sorting tutorial, see Sorting HOW TO.