input: 
  class Rectangle:
      """A class to represent a two-dimensional rectangle with a length and width.
      
      >>> rect = Rectangle(4, 5)
      >>> rect.area()
      20
      >>> rect2 = Rectangle(7, 3)
      >>> rect2.area()
      21
      """
      def __init__(self, length, width):
          self.length = length
          self.width = width
  
      def area(self):
          return self.length * self.width
  
output:     def perimeter(self):
          return 2 * (self.length + self.width)