Q&A

What is the difference between sort() and sorted() in Python?

By AlgoInsight Team2026-06-07algorithmsstringspython

Overview

Python offers two ways to sort, and confusing them causes a very common bug. The core distinction: sort() mutates a list and gives back nothing; sorted() leaves the input alone and gives back a new list. Choosing correctly depends on whether you need the original preserved and whether your data is even a list.

graph TD
    A[Need to sort] --> B{Keep the original?}
    B -->|no, mutate in place, it's a list| C["list.sort() → returns None"]
    B -->|yes, want a new sorted copy| D["sorted(iterable) → returns new list"]
    D --> E[Works on ANY iterable]
    style D fill:#1e1b4b,color:#fff

list.sort() - in place, returns None

nums = [3, 1, 2]
result = nums.sort()      # sorts nums IN PLACE
print(nums)               # [1, 2, 3]  - original changed
print(result)             # None       - sort() returns None!

Because it returns None, the classic mistake is nums = nums.sort() - that overwrites your list with None.

sorted() - new list, original untouched

nums = [3, 1, 2]
ordered = sorted(nums)    # returns a NEW sorted list
print(ordered)            # [1, 2, 3]
print(nums)               # [3, 1, 2]  - original unchanged

sorted() works on any iterable and always produces a list:

sorted("dcba")            # ['a', 'b', 'c', 'd']  (string → list)
sorted((3, 1, 2))         # [1, 2, 3]              (tuple → list)
sorted({3, 1, 2})         # [1, 2, 3]              (set → list)
sorted({"b": 2, "a": 1})  # ['a', 'b']             (dict → keys sorted)

Shared Parameters: key and reverse

Both accept the same options:

words = ["banana", "kiwi", "apple"]

sorted(words, key=len)              # ['kiwi', 'apple', 'banana'] (by length)
sorted(words, reverse=True)         # descending
words.sort(key=str.lower)           # case-insensitive, in place

# sort objects by an attribute / dict by a field:
people.sort(key=lambda p: p["age"])
sorted(records, key=lambda r: (r["dept"], -r["salary"]))   # multi-key

key takes a function applied to each element to derive the comparison value; reverse=True flips the order.

Comparison Table

Stability

Both are stable - elements that compare equal keep their original relative order. This lets you sort by multiple criteria with successive sorts (sort by secondary key first, then primary).

When to Use Which

• sort() - you have a list, don't need the original, and want to avoid the memory of a copy.
• sorted() - you need to keep the original, you're sorting a non-list iterable (tuple, string, dict, generator), or you want to sort inside an expression.

Why This Matters in Interviews

This is a frequent Python question precisely because of the None gotcha. Nail: sort() mutates in place and returns None (so never do x = x.sort()); sorted() returns a new list and works on any iterable; both share key/reverse and are stable Timsort. Demonstrating key with a lambda (and multi-key tuples) and explaining stability shows fluency.

Common Pitfalls

• nums = nums.sort() → nums becomes None (the #1 mistake).
• Calling .sort() on a non-list (tuples/strings have no .sort() - use sorted()).
• Expecting sorted() to modify the original (it doesn't).
• Forgetting that sorted() of a dict sorts its keys by default.
• Passing a comparison value instead of a key function (Python 3 dropped cmp; use key or functools.cmp_to_key).

References

• Python sorted(): https://docs.python.org/3/library/functions.html#sorted
• Python Sorting HOW TO: https://docs.python.org/3/howto/sorting.html

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