Python TypeError: 'NoneType' object is not callable

 

I am mere a beginner of Python. I am getting the following error and suspect that it has something to do with the dictionary I've used as switch case (since python does not provide switch ). Following is my code:

class Arithmetic:

    a,b,choice = 0,0,0


    def __init__(self):

        print "\n\n"

        for num in range(30):
            print "*",

        print "\n"    

        print "Welcome to CLC (Command Line Calculator)"

        print "\n"

        for num in range(30):
            print "*",

        print "\n"

    def menu(self):

        print "1. Add"
        print "2. Substract"
        print "3. Multiply"
        print "4. Divide"
        print "5. Modulo"
        print "6. Exit \n\n"

        self.choice = raw_input("Enter Your Choice: ")

        if self.choice == '0':
            exit("Thank you for using the program")

        selector = {
                "1" :   self.add(),
                "2" :   self.substract(), 
                "3" :   self.multiply(),
                "4" :   self.divide(),
                "5" :   self.modulo()
                }

        selector[self.choice]()                    

    def add(self):
        print "Add called"

    def substract(self):
        print "Substract called"

    def multiply(self):
        print "Multiply called"

    def divide(self):
        print "Divide called"

    def modulo(self):
        print "Modulo called"


    def main(self):

        while self.choice != '6':
            self.menu()   



a = Arithmetic()
a.menu() 

The Error is:

When you do

self.add()

you are calling the method (you will get a result). If you want to specify the method, delete the ():

selector = {
            "1" :   self.add,
            "2" :   self.substract, 
            "3" :   self.multiply,
            "4" :   self.divide,
            "5" :   self.modulo
            }

Traceback (most recent call last):
 File "arithmetics.py", line 75, in <module>
a.menu()
File "arithmetics.py", line 43, in menu
  selector[self.choice]()                    
TypeError: 'NoneType' object is not callable