>>> class Rectangle(object):
...   @property
...   def area(self):
...     return self.length * self.width
... 
>>> r = Rectangle()
>>> r.length, r.width = 100, 500
>>> r.area
50000
>>> class Rectangle2(Rectangle):
...   def area2(self):
...     return self.length * self.width
... 
>>> r = Rectangle2()
>>> r.length, r.width = 100, 500
>>> r.area
50000
>>> r.area2
<bound method Rectangle2.area2 of <__main__.Rectangle2 object at 0x7fdbfb964c10>>
>>> r.area2()
50000