Python TypeError: 'NoneType' object is not callable

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本文探讨了Python中没有内置Switch Case语句的问题，并提供了一种使用字典来模仿Switch Case的解决方案。作者在尝试实现一个命令行计算器时遇到了错误，通过调整字典中的方法引用而非调用，成功解决了TypeError。

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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()

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

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
            }