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

Python provides two ways to sort a list, the built-in list method list.sort() and the built-in function sorted(). Although both will sort the elements of a list, if used incorrectly they can produce unexpected or undesired results.

Differences and similarities

The primary difference between the two is that list.sort() will sort the list in-place, mutating its indexes and returning None, whereas sorted() will return a new sorted list leaving the original list unchanged. Another difference is that sorted() accepts any iterable while list.sort() is a method of the list class and can only be used with lists.

nums = [2, 3, 1, 5, 6, 4, 0]

print(sorted(nums))   # [0, 1, 2, 3, 4, 5, 6]
print(nums)           # [2, 3, 1, 5, 6, 4, 0]

print(nums.sort())    # None
print(nums)           # [0, 1, 2, 3, 4, 5, 6]

Both list.sort() and sorted() have the same key and reverse optional arguments and can be called on each list element prior to making comparisons.

When to use each one

list.sort() should be used whenever mutating the list is intended and retrieving the original order of the elements is not desired. On the other hand, sorted() should be used when the object to be sorted is an iterable (e.g. list, tuple, dictionary, string) and the desired outcome is a sorted list containing all elements.