input: Write a function to sort a list of strings by their length in ascending order. The function should return the sorted list. 
  
  def sort_by_length(strings):
      for i in range(len(strings) - 1):
          for j in range(i + 1, len(strings)):
              if len(strings[i]) > len(strings[j]):
                  strings[i], strings[j] = strings[j], strings[i]
      return strings
  
  assert sort_by_length(["cat", "dog", "apple", "banana"])==["cat", "dog", "apple", "banana"]
  assert sort_by_length(["elephant", "mouse", "ant"])==["ant", "mouse", "elephant"]
  assert sort_by_length(["", "a", "bb", "ccc"])==["", "a", "bb", "ccc"]
  assert sort_by_length([])==[]
  assert sort_by_length(["python", "java", "ruby"])==["java", "ruby", "python"]
output: def sort_by_length(strings):
      return sorted(strings, key=len)