In Python 3 the cmp built-in function was removed, together with the __cmp__ special method.

From the documentation:

The cmp() function should be treated as gone, and the cmp() special method is no longer supported. Use lt() for sorting, eq() with hash(), and other rich comparisons as needed. (If you really need the cmp() functionality, you could use the expression (a > b) - (a < b) as the equivalent for cmp(a, b).)

Moreover all built-in functions that accepted the cmp parameter now only accept the key keyword only parameter.

In the functools module there is also useful function cmp_to_key(func) that allows you to convert from a cmp-style function to a key-style function:

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.