Python cmp-to-key()

The cmp_to_key() function from functools is a higher-order function that converts an old-style comparison function into a key function usable with tools like sorted(), min(), or max(). A comparison function is any callable that takes two arguments and returns:

• A negative number if the first argument is less than the second,
• A positive number if the first argument is greater,
• Or zero if they are equal.

A key function, by contrast, takes a single argument and returns a value to be used as a sorting key. cmp_to_key() is especially useful when migrating code from Python 2 (which supported comparison functions) to Python 3 (which only supports key functions).

Syntax

The cmp-to-key() function is part of the functools module:

from functools import cmp_to_key

functools.cmp_to_key(comp_func)

Parameters:

comp_func: This is a function that takes two arguments and compares them, returning:

• A negative number, if the first value is less than the second
• A positive number, if the first value is more than the second
• Or zero, if both values are equal

Return value:

Returns a key function that can be used as the key argument in sorting functions like sorted(), list.sort(), min(), or max().

Example 1

This example demonstrates checking which of two values is heavier (larger) using cmp_to_key():

from functools import cmp_to_key

# Create a list of numbers
weights = [20, 200, 10, 1]

# Define the comparison function
def lighter_to_heavier(a: float, b:float):
  if a == b: return 0
  return 1 if a > b else -1

# Reorganize the list from smallest to largest
print("Unsorted weights", weights)
weights.sort(key=cmp_to_key(lighter_to_heavier))
print("Sorted weights", weights)

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The output of this code is:

Unsorted weights [20, 200, 10, 1]
Sorted weights [1, 10, 20, 200]

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Example 2

This example reorganizes the list from largest to smallest by modifying the comparison function:

from functools import cmp_to_key

# Create a list of numbers
weights = [20, 200, 10, 1]

# Define the comparison function
def heaviest_to_smallest(a: float, b: float):
  if a == b: return 0
  return 1 if a < b else -1

# Reorganize the list from largest to smallest
print("Unsorted weights", weights)
weights.sort(key=cmp_to_key(heaviest_to_smallest))
print("Sorted weights", weights)

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The output of this code is:

Unsorted weights [20, 200, 10, 1]
Sorted weights [200, 20, 10, 1]

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Codebyte Example: Reorganize a list of strings by length

This example shows sorting strings by their length using a comparison function converted with cmp_to_key():

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