Master Python’s Stable Sorting: Using sorted() with key and reverse

Sorting in Python seems simple until you notice some quirks.

sorted() is stable

The sorted() function is stable, meaning that when two elements compare equal they retain their original order, which is useful for multi‑criteria sorting.

For example, to sort a list of people first by age then by name you can perform two sorts leveraging stability:

<code>people = [
    {"name": "Alice", "age": 25},
    {"name": "Bob", "age": 30},
    {"name": "Charlie", "age": 25}
]

# First sort by name
people.sort(key=lambda x: x["name"])
# Then sort by age
people.sort(key=lambda x: x["age"])

print(people)
</code>

In the final list, Alice and Charlie both have age 25, but because the name sort was applied first, Alice remains before Charlie.

Using key parameter for custom sorting — more flexible than you think

The key parameter of sorted() can accept any function that returns a value, allowing you to combine and transform values for advanced sorting.

For instance, to sort strings by length and then alphabetically you can create a tuple in the key function:

<code>words = ["banana", "apple", "pear", "blueberry", "kiwi"]
sorted_words = sorted(words, key=lambda x: (len(x), x))
print(sorted_words)
</code>

Here the key function first sorts by len(x) and then by the string itself, which is handy when you need multi‑criteria sorting without multiple calls to sorted() .

Additional tip: Easy reverse sorting

The sorted() function has a reverse parameter; setting it to True yields descending order without modifying the key function.

<code>numbers = [4, 2, 9, 1]
sorted_numbers = sorted(numbers, reverse=True)
print(sorted_numbers)
</code>

The sorted() function in Python is powerful: from its stability to the flexibility of the key parameter and the convenience of reverse , it enables you to handle complex sorting tasks easily and avoid unexpected behavior.