class FirstClass:              # define a class object
    def setdata(self, value):  # define class methods
        self.data = value      # self is the instance
    def display(self):
        print self.data        # self.data: per instance
 

class SecondClass(FirstClass):        # inherits setdata
    def display(self):                # changes display 
        print 'Current value = "%s"' % self.data


class ThirdClass(SecondClass):         # isa SecondClass
    def __init__(self, value):         # "ThirdClass(x)"
        self.data = value
    def __add__(self, other):          # "self + other"
        return ThirdClass(self.data + other)
    def __mul__(self, other):
        self.data = self.data * other  # "self * other"