How to call class method within namespace of the same class in python 3.x

Question

When working with python instances, it is possible to access bound methods of the same class using self. This resolves to a method corresponding to the same class in hierarchy.

class A:
    def f(self):
        return 1

    def __init__(self):
        self.v = self.f()

class B(A):
    def f(self):
        return 2

b = B()
# b.v is set to 2

But, when working with class methods, there is no apparent way of accessing methods of the same class as above.

In my use case, f above needs to be a class method and I need to set class variable v according to f corresponding to the same class. Somewhat like:

class A:
    @classmethod
    def f(cls):
        return 1

    v = resolution_of_calling_class.f()

class B(A):
    @classmethod
    def f(cls):
        return 2

# B.v should be 2

edit: v is actually an attribute defined by parent class, which should find a method overridden by child class

I don't think that I understand your use case; perhaps you could make sure to clarify in the post when you are referring to a superclass or a subclass. — Josh Katofsky
– Josh Katofsky, Commented Nov 11, 2019 at 7:04
Sorry for the ambiguity in original question. I've tried to make it more clear. I'm trying to find the resolution_of_calling_class as described in the second snippet, similar to self in the first snippet refers to calling class. — Pushkar Nimkar
– Pushkar Nimkar, Commented Nov 11, 2019 at 8:11

trsvchn · Accepted Answer · 2020-05-11 09:01:42Z

1

You just need to override __new__ method, since it is invoked before the __init__ and its purpose is to create an instance, that will be initialized by __init__.

class A:

    def __new__(cls, *args, **kwargs):
        cls.v = cls.f()
        return super().__new__(cls, *args, **kwargs)

    @classmethod
    def f(cls):
        return 1


class B(A):
    @classmethod
    def f(cls):
        return 2


a = A()
print(a.v)

b = B()
print(b.v)

1
2

edited May 11, 2020 at 9:01

answered May 11, 2020 at 8:49

trsvchn

9,1113 gold badges26 silver badges35 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Pushkar Nimkar Over a year ago

Hi, that's an useful answer! I think I implemented this using Mixin pattern in my actual implementation. But this really answers the question!

trsvchn Over a year ago

@PushkarNimkar Nice!

Dr Code Monkey · Accepted Answer · 2019-11-11 07:05:31Z

0

I am not 100% sure I understand what you are trying to do.

I used your code above and

class A:
    @classmethod
    def f(cls):
        return 1

class B:
    @classmethod
    def f(cls):
        return 2

print(B.f())

gives me 2 just as I expected it would. Should B be a child class of A as in the first example?

answered Nov 11, 2019 at 7:05

Dr Code Monkey

361 silver badge6 bronze badges

1 Comment

Pushkar Nimkar Over a year ago

Yes B should be a child class of A as in the first example.

Collectives™ on Stack Overflow

How to call class method within namespace of the same class in python 3.x

2 Answers 2

2 Comments

1 Comment

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Related