在Python中返回意外的无值

Question

def sort_by_length(words):
    if words!=None:
        words= words.sort(key=len, reverse=True)
    return words

def single_sort_by_len_test(input, result):
    assert result == sort_by_length(list(input))

single_sort_by_len_test(["dog", "apple", "bear"], ["apple", "bear", "dog"])

这里的单词返回None值。但答案应该是["apple", "bear", "dog"]。无法看出问题所在。