input: def sort_by_length(strings):
      """ Sorts a list of strings by their length.
      >>> sort_by_length(["cat", "dog", "apple", "banana"])
      ['cat', 'dog', 'apple', 'banana']
      >>> sort_by_length(["hello", "world", "a", "is"])
      ['a', 'is', 'world', 'hello']
      """
output:     return sorted(strings, key=len)