list.sort() returns None

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Accepted Answer

list.sort() sorts the list in place and returns None, not the sorted list. Assigning the return value of .sort() is one of the most common Python mistakes - you silently get None while your data disappears. The list itself (nums) is sorted correctly to [1, 1, 3, 4, 5]. If you need the sorted result as a value you can pass around, use sorted(nums) instead.