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sort() vs sorted()

list.sort() and sorted() look similar, but they behave differently in an important way. One mutates the original list and returns None. The other returns a new sorted result.

What is happening?​

numbers = [3, 1, 2]
result = numbers.sort()

print(numbers)
print(result)

Output:

[1, 2, 3]
None

What you might expect: result holds the sorted list.

What actually happens: sort() changes numbers in place and returns None.

Why this matters​

This design helps make mutation explicit. It prevents code from accidentally treating an in-place operation like a pure one.

The bug usually appears when code writes:

numbers = [3, 1, 2]
sorted_numbers = numbers.sort()

Now sorted_numbers is None.

Choose based on whether mutation is intended​

Use sort() when you intentionally want to mutate the existing list:

numbers.sort()

Use sorted() when you want a new sorted value:

numbers = [3, 1, 2]
sorted_numbers = sorted(numbers)

Rules of thumb​

  • sort() mutates a list and returns None.
  • sorted() returns a new sorted result.
  • Use sort() for intentional in-place mutation.
  • Use sorted() when you want to preserve the original data.