How to Sort a List in Python: key, Stable Ties and In-Place sort

Start with a small, explicit task

Order fictional tasks by ascending priority number and then by task name. Compare that with sorting only by priority, where the two priority-two tasks should keep their original order. Check that creating a sorted result does not reorder the source list.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_sort_list.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

tasks = [
    {"task": "seal", "priority": 2},
    {"task": "label", "priority": 1},
    {"task": "archive", "priority": 2},
]
ordered = sorted(tasks, key=lambda item: (item["priority"], item["task"]))
assert [item["task"] for item in ordered] == ["label", "archive", "seal"]
assert [item["task"] for item in tasks] == ["seal", "label", "archive"]
returned = tasks.sort(key=lambda item: item["priority"])
assert returned is None
assert [item["task"] for item in tasks] == ["label", "seal", "archive"]
assert sorted([]) == []
print([item["task"] for item in ordered])
print([item["task"] for item in tasks])
print(returned)

Explain the result

The tuple comparison first uses priority and then the task name when priorities tie. label comes first at priority one; archive precedes seal alphabetically at priority two. The first source-order assertion confirms that sorted returned a new outer list instead of reordering tasks.

The in-place sort uses only priority. seal and archive now have equal keys, so their original relative order is retained. That stable behavior is different from an explicit alphabetical tie-break. Both outer lists still reference the same task dictionaries; sorting does not create deeply independent record copies.

Avoid these mistakes

Do not assign tasks = tasks.sort(), which replaces the name with None after sorting the former list. Make the direction of each key explicit: reverse=True reverses the overall order, so mixed ascending/descending requirements need a carefully designed key or multiple deliberate stable passes.