Python cmp() Function: Compare Two Objects – Deprecated in Python 3

Example (Python 2)

Let’s look at a simple example to see how it behaved.

print(cmp(5, 3))   # Output: 1
print(cmp(3, 5))   # Output: -1
print(cmp(4, 4))   # Output: 0

Explanation: The function returns a number based on how the two values compare.

How to Do This in Python 3

Since this function is no longer available, here’s the modern approach. Since cmp() is not available in Python 3, you can use a simple expression to get similar behavior:

x = 5
y = 3

result = (x > y) - (x < y)
print(result)  # Output: 1

Explanation: (x > y) becomes 1 if true, otherwise 0 (x < y) becomes 1 if true, otherwise 0 Subtracting them gives the same result as cmp().

Why Was cmp() Removed?

There were a few reasons why Python moved away from this function.

• Python now prefers direct comparison operators like <, >, and ==
• Sorting is handled using key functions instead of comparison functions
• Removing it simplified the language and avoided confusion between Python 2 and Python 3

How to Replicate cmp() Behavior in Python 3

Although the cmp() function is no longer available in Python 3, you can still achieve the same result using a simple expression. The idea is to compare two values and convert the result into -1, 0, or 1.

x = 5
y = 3

result = (x > y) - (x < y)
print(result)  # Output: 1

Explanation: (x > y) returns True (1) if x is greater than y (x < y) returns True (1) if x is less than y Subtracting them gives:

• 1 → when x > y
• 0 → when x == y
• -1 → when x < y

This produces the same behavior as the old cmp(x, y) function.

Using cmp()-like Logic in Sorting

If you previously used cmp() for sorting, Python 3 provides a better approach using key functions.

data = [5, 2, 9, 1]

# Sorting using key instead of cmp
sorted_data = sorted(data)
print(sorted_data)  # Output: [1, 2, 5, 9]

For custom sorting logic, you can use functools.cmp_to_key() to convert a comparison function into a key function.

from functools import cmp_to_key

def compare(a, b):
    return (a > b) - (a < b)

data = [5, 2, 9, 1]
sorted_data = sorted(data, key=cmp_to_key(compare))

print(sorted_data)  # Output: [1, 2, 5, 9]

Explanation:

This example demonstrates how to use comparison-based logic in Python 3, even though the cmp() function is no longer available.

The compare(a, b) function is designed to behave like the old cmp() function. It returns:

• 1 when a > b
• 0 when a == b
• -1 when a < b

However, Python 3 does not accept comparison functions directly in sorting. Instead, it requires a key function.

This is where cmp_to_key() is used. It converts the compare() function into a key-based format that works with sorted().

As a result, the list is sorted using the custom comparison logic, and the output is [1, 2, 5, 9].