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, i.e. 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

cls(userId) instantiates your class, and userId gets passed to __init__. So for User, the userId will be stored in the new instance. — khelwood
– khelwood, Commented Oct 12, 2018 at 14:30
cls(userId) works exactly the same as if you did User(userId), and where that value gets stored from there works exactly the same too… — deceze
– deceze ♦, Commented Oct 12, 2018 at 14:30
in python 3 class is defined as such class User: without parenthesis unless if you want this class to inherent from another class then you use parenthesis class User(User2): — Khalil Al Hooti
– Khalil Al Hooti, Commented Oct 12, 2018 at 14:34

Zags · Accepted Answer · 2018-10-12 14:40:09Z

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.

answered Oct 12, 2018 at 14:40

Zags

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4 Comments

TheRealFakeNews Over a year ago

then why not return User(userId)?

Zags Over a year ago

In the case of your example, returning cls(userId) and User(userId) is identical. The difference only becomes apparent in a child class of User.

TheRealFakeNews Over a year ago

and what would that difference be?

Zags Over a year ago

The difference is that cls refers to the current class, where User refers to the User class in particular. If you have a child class SpecialUser(User), using cls in the createUser method means SpecialUser.createUser will return a SpecialUser instead of a User

Olivier Darrouzet · Accepted Answer · 2018-10-12 14:34:26Z

0

The userId parameter is stored in the __init__ method.

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

answered Oct 12, 2018 at 14:34

Olivier Darrouzet

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Comments

blue note · Accepted Answer · 2018-10-12 14:37:51Z

0

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.

answered Oct 12, 2018 at 14:37

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