How do class methods affect instance variables in Python?

Question

I've read throughly many posts here on the difference between class methods and instance methods. I understand them conceptually, but now I'm trying to figure out the more subtle nuances. In the example below, if I call User.createUser('xyz') , where does userId get stored? does it go to (*) below, ie on the instance? Or would it be as if I inserted userId = None where I have the comment # placeholder and the userId passed in from User.createUser('xyz') then assigns the class variable userId with 'xyz' ?

class User():

    # placeholder


    def __init__(self, userId):
        self.userId = userId # (*)

    @classmethod
    def createUser(cls, userId):
        if not isValid(userId): # isValid may or may not be part of the class
            return False
        else:
            return cls(userId)

    # ... other methods

Answer 1

Your classmethod createUser returns a new instance of the User object. The parameter passed to it is stored as an attribute on this new User instance.

Answer 2

The userId parameter is stored in the __init__ method.

The call to cls(userId) is equivalent to a call to User(userId)

Answer 3

In your case, neither. The return cls(userId) returns a new object*, and the userId is assigned to the self of the new object. So, it's neither a class nor an instance variable, it's an instance variable of another object.