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functools.cmp_to_key

functools

Convert an old-style cmp(a, b) comparison function into a key function for sorted().

Syntax

functools.cmp_to_key(func)

Reach for it only when the ordering cannot be expressed as a single key because the decision depends on a pair of elements at once — the classic case is arranging numbers to form the largest possible number. Otherwise a plain key= wins: the key is computed once per element, while a comparison function is called about n log n times through a wrapper object. Also, cmp must return a negative number, zero or a positive number — not True/False.

Examples

import functools
def cmp(a, b):
    return a - b
print(sorted([3, 1, 2], key=functools.cmp_to_key(cmp)))   # → [1, 2, 3]

See also

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