input: def sort_by_length(strings):
      """
      Sort a list of strings by their length in ascending order.
      >>> sort_by_length(["cat", "dog", "apple", "banana"])
      ["cat", "dog", "apple", "banana"]
      >>> sort_by_length(["short", "verylongstring", "medium", "long"])
      ["short", "medium", "long", "verylongstring"]
      """
output:     return sorted(strings, key=len)