subclass.py

class Shape:
    pass

class Rectangle(Shape):
    def __init__(self, length, width):
        self.length = length
        self.width = width
        self.area = length * width
    
    def get_area(self):
        return self.length * self.width
        
class Square(Rectangle):
    def __init__(self,length):
        Rectangle.__init__(self,length,length)