Master Python Sorting: From Simple Lists to Custom Key Functions

Python offers several flexible ways to sort data, ranging from simple numeric lists to complex structures such as dictionaries and tuples. The article covers the built‑in sorted() function, the list .sort() method, key‑based sorting for complex types, multi‑criteria sorting, and custom comparison functions using functools.cmp_to_key.

Using sorted()

The sorted() function accepts any iterable and returns a new sorted list without modifying the original data.

numbers = [5, 2, 9, 1, 5, 6]
sorted_numbers = sorted(numbers)
print(sorted_numbers)  # Output: [1, 2, 5, 5, 6, 9]

# Descending order
sorted_numbers_desc = sorted(numbers, reverse=True)
print(sorted_numbers_desc)  # Output: [9, 6, 5, 5, 2, 1]

Using .sort() method

The list method .sort() sorts the list in place, altering the original list.

numbers = [5, 2, 9, 1, 5, 6]
numbers.sort()
print(numbers)  # Output: [1, 2, 5, 5, 6, 9]

# Descending order
numbers.sort(reverse=True)
print(numbers)  # Output: [9, 6, 5, 5, 2, 1]

Sorting complex data types

When sorting lists of dictionaries or tuples, the key parameter can extract a sortable value from each element.

people = [
    {'name': 'Alice', 'age': 30},
    {'name': 'Bob', 'age': 25},
    {'name': 'Charlie', 'age': 35}
]
# Sort by age ascending
sorted_people_by_age = sorted(people, key=lambda person: person['age'])
print(sorted_people_by_age)

# Sort by name descending
sorted_people_by_name_desc = sorted(people, key=lambda person: person['name'], reverse=True)
print(sorted_people_by_name_desc)

students = [
    ('John', 'A', 15),
    ('Jane', 'B', 12),
    ('Dave', 'B', 10)
]
# Multi‑criteria: first by grade, then by age
sorted_students = sorted(students, key=lambda s: (s[1], s[2]))
print(sorted_students)

Custom comparison function (Python 3)

Python 3 no longer supports the cmp argument directly, but you can achieve similar behavior with functools.cmp_to_key.

from functools import cmp_to_key

def compare_items(x, y):
    return x - y  # Simple numeric comparison

numbers = [5, 2, 9, 1, 5, 6]
sorted_numbers = sorted(numbers, key=cmp_to_key(compare_items))
print(sorted_numbers)

Choose the method that best fits your specific sorting needs—whether you need a new list, in‑place modification, sorting by complex keys, or custom comparison logic—to write concise and efficient Python code.