Programming Fundamentals › Collections

Sorting with a Key or Comparator

Sorting by a custom key or comparison function.

Also known as: sort by key, custom sort, comparator

Sorting a list of numbers or strings is one call. Real data is usually objects, so you tell the sort what to sort by, using a key function or a comparator.

users = [{"name": "Bo", "age": 31}, {"name": "Ana", "age": 25}]

sorted(users, key=lambda u: u["age"])                    # by age
sorted(users, key=lambda u: u["age"], reverse=True)      # descending
sorted(users, key=lambda u: (u["age"], u["name"]))       # by age, then name
users.sort(key=...)                                      # in place; returns None

users.sort((a, b) => a.age - b.age);          // comparator: negative, zero or positive
users.sort((a, b) => a.name.localeCompare(b.name));

A key function returns the value to compare. A comparator takes two items and says which comes first.

Gotchas

• JavaScript sorts as strings by default. [10, 9, 1].sort() gives [1, 10, 9]. Always pass a comparator for numbers.
• sort() changes the array in JavaScript, and in Python list.sort() returns None. sorted() returns a new list.
• Stability. A stable sort keeps equal items in their original order, so sorting by name and then by age gives a name-sorted list within each age (stable sort). Python’s sort is stable, and modern JavaScript’s is too.
• Case, accents and languages. Plain string comparison orders by character codes ("Z" < "a"). Use locale-aware comparison for people-facing lists.
• Missing values (None, null) can raise errors or sort oddly. Handle them in the key.
• Comparators must be consistent, or results are unpredictable.
• Sorting costs about n log n. Don’t re-sort inside a loop (big O).

For huge data, sort in the database (ORDER BY) instead of in your program.