How to define a callable that can only be called once?

Question

To demonstrate what I want to do, here's a piece of code:

class CallOnce(object):
    called=False
    def web_service(cls,macid):
        if cls.called:
            print ("already called")
            return
        else:
            # do stuff
            print ("called once")
            cls.called = True
            return macid

To test our class, I proceed as follows:

for i in range(2):

    macid = "123"
    call_once_object = CallOnce()
    call = call_once_object.web_service(macid)
    print(call)

The expected result should be like this:

called once
123
already called

Except I got this as a result:

called once
123
called once
123

The idea is to store the value 123 only once in the call variable without using global variable.

Answer 1

cls.called is an attribute of the instance, not the class. Each time you create a new object, it gets its own attribute, which defaults to False .

If you want to share this among all the instances, you should use CallOnce.called , not cls.called .

BTW, the conventional name for the first argument of instance methods is self . cls is used for class methods.

Answer 2

So you're trying to save some state. What you could do is use an object instead.

class Test():
    def __init__(self):
        self.called = False

    def call_me_maybe(self):
        if (not self.called):
            print('Hey, you called?')
            self.called = True
        else:
            print("Stop calling me it's getting weird")

test = Test()
test.call_me_maybe() #-> Hey, you called?
test.call_me_maybe() #-> Stop calling me it's getting weird

Answer 3

You don't need a class for this. Functions can have their own attributes.

def web_service(macid):
    if hasattr(web_service, 'called'):
        print ("already called")
        return
    else:
        # do stuff
        print ("called once")
        web_service.called = True
        return macid

web_service(5)
web_service(6)      

Output:

called once
already called