Just a stupid calculator

calc.py

# -*- coding: utf-8 -*-
"""
Created on Thu Dec 19 18:15:29 2013

@author: Tomasz
"""

import math, os

class Calc():    
    def __init__(self):
        self.operations = {
            "+" : lambda num1, num2: num1 + num2,
            "-" : lambda num1, num2: num1 - num2,
            "/" : lambda num1, num2: num1 / num2,
            "*" : lambda num1, num2: num1 * num2,
            "sqrt" : lambda num1, num2: math.sqrt(num1),
        }
    
    def main(self):
        print "Simple python calculator"
        print "Supports +, -, /, * and sqrt %s" % os.linesep
        while True:
            key, op = self.get_operation()
            num1 = self.user_input()
            if key == "sqrt":
                print "%s %r = %r" % (key, num1, op(num1, 0))
            else:
                num2 = self.user_input()
                print "%r %s %r = %r" % (num1, key, num2, op(num1, num2))       
            if not self.try_again():
                break
            
    def user_input(self):
        while True:
            try:
                return input("Number: ")
            except ValueError:
                print "Didn't understand that"
        
    def get_operation(self):
        while True:
            op = raw_input("Enter your operation: ").strip().lower()
            try:
                return op, self.operations[op]
            except KeyError:
                print "%s is not a known operation" % op
        
    def try_again(self):
        while True:
            again = raw_input("Again? Yes/No: ").strip().lower()
            if again in "yes":
                return True
            if again in "no":
                return False
            else:
                print "Didn't understand %s" % again
    

if __name__ == '__main__':
    Calc().main()