#!/usr/bin/env python3
# demonstrating class definitions
class calculator:
    def __init__(self,num1,num2):
        self.num1 = num1
        self.num2 = num2
    
    def add(self,n1=None,n2=None):
        if n1 == None:
            self._t1 = self.num1
        else:
            self._t1 = n1
        if n2 == None:
            self._t2 = self.num2
        else:
            self._t2 = n2
        return self._t1 + self._t2
    
    def sub(self):
        return self.num1-self.num2
    def mul(self, n1,n2):
        return n1*n2
    def div(self, n1,n2):
        return n1*1.0/n2
    def num1(self):
        return num1
    def num2(self):
        return num2
    
if __name__ == "__main__":
    calc = calculator(4,5)
    print('{} + {} = {}'.format(calc.num1,calc.num2,calc.add()))
    print('{} - {} = {}'.format(calc.num1,calc.num2,calc.sub()))
    print('Mul is', calc.mul(8,9))
    print('div is', calc.div(8,9))