main.py

# Program make a simple calculator that can add, subtract, multiply and divide using functions

# This function adds two numbers 
def add(x, y):
   return x + y

# This function subtracts two numbers 
def subtract(x, y):
   return x - y

# This function multiplies two numbers
def multiply(x, y):
   return x * y

# This function divides two numbers
def divide(x, y):
   return x / y

while True:
    print "Select operation."
    print "1.Add"
    print "2.Subtract"
    print "3.Multiply"
    print "4.Divide"
    print "5. exit"

    # Take input from the user 
    choice = raw_input("Enter choice(1/2/3/4):")

    num1 = input("Enter first number: ")
    num2 = input("Enter second number: ")

    if choice == '1':
        print num1,"+",num2,"=", add(num1,num2)

    elif choice == '2':
        print num1,"-",num2,"=", subtract(num1,num2)

    elif choice == '3':
        print num1,"*",num2,"=", multiply(num1,num2)

    elif choice == '4':
        print num1,"/",num2,"=", divide(num1,num2)
    elif choice == '5':
        print "Bye"
        break
    else:
        print 'Invalid Option'