When to use

Python's built-in sorted()/list.sort() is Timsort — O(n log n), stable, and almost always the right choice.

Rules

Use key= for custom ordering
For top-k elements, use heapq.nlargest/nsmallest (O(n log k)) instead of full sort
For finding just the kth element, consider quickselect or statistics.median
Counting sort / radix sort help only when values are bounded integers
When the problem says "sort by X then by Y," use a tuple key: key=lambda x: (x.a, x.b)
For reverse on one field only, negate it or use functools.cmp_to_key
ALWAYS prefer built-in sort — it's optimized and stable
NEVER implement your own sort algorithm unless the problem requires it

Timsort: O(n log n). heapq.nlargest/nsmallest: O(n log k).