ReviseAlgo Logo

Built-in Functions

sorted and reversed

Sorting iterables with custom keys and reversing sequences

Interview: Very common — stable sort, multi-key sorting, and custom key functions are frequently tested

Last Updated: June 12, 2026 • 8 min read

sorted() returns a new sorted list from any iterable, while reversed() returns an iterator that yields elements in reverse order. Python's sort uses Timsort — a stable, adaptive algorithm that is O(n log n) worst case but very fast on real-world data.

sorted() Function

  • Returns new list: Unlike list.sort() which sorts in-place, sorted() always creates a new list
  • Works on any iterable: Lists, tuples, strings, dicts (keys), generators — all work
  • key parameter: A function called on each element for comparison — key=len, key=str.lower
  • reverse parameter: reverse=True for descending order
  • Stable sort: Equal elements maintain their original relative order — crucial for multi-key sorting

Multi-Key Sorting

  • Tuple key: key=lambda x: (x.age, x.name) — sorts by age first, then name
  • Mixed order: key=lambda x: (-x.score, x.name) — descending score, ascending name (for numbers)
  • Multiple passes: Sort by secondary key first, then primary — works because sort is stable

reversed() Function

  • Returns iterator: Not a list — use list(reversed(seq)) if you need a list
  • Works on sequences: Lists, tuples, strings, range — anything with __reversed__ or indexing
  • vs slicing: reversed(lst) returns iterator; lst[::-1] returns a new list

sorted() vs list.sort()

sorted(iterable) works on any iterable and returns a new list. list.sort() only works on lists but modifies in-place (slightly faster, no extra memory). Use sorted() by default; use .sort() only when you specifically want in-place sorting.

Use Cases

Sorting complex data structures by multiple criteria

Ranking and leaderboard generation with multi-key sorting

Data cleaning and preprocessing pipelines

Displaying sorted results in APIs and reports

Implementing priority queues and ordered collections

Common Mistakes

Using list.sort() and assigning the result — it returns None, not the sorted list

Forgetting that sorted() returns a list even for strings/tuples input

Not knowing sort is stable — you can rely on equal elements keeping their order

Using reverse=True when you need mixed ascending/descending — use negation (-x) for numbers

Forgetting that reversed() returns an iterator, not a list — need list() to materialize