Python: How to call an instance method from a class method of the same class

Question

I have a class as follows:

class MyClass(object):
    int = None
    def __init__(self, *args, **kwargs):
        for k, v in kwargs.iteritems():
            setattr(self, k, v)

    def get_params(self):
        return {'int': random.randint(0, 10)}

    @classmethod
    def new(cls):
        params = cls.get_params()
        return cls(**params)

and I would like to be able to do:

>>> obj = MyClass.new()
>>> obj.int  # must be defined
9

I mean without creating a new instance of MyClass, but obviously it’s not that simple, because calling MyClass.new() throws TypeError: unbound method get_params() must be called with MyClass instance as first argument (got nothing instead)

Is there any way to accomplish so?
Thanks.

Asked By: Gerard

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Source

Answer 1

No, you can’t and shouldn’t call an instance method from a class without an instance. This would be very bad. You can, however call, a class method from and instance method. Options are

make get_param a class method and fix references to it
have __init__ call get_param, since it is a instance method

Also you may be interested in an AttrDict since that looks like what you are trying to do.

Answered By: cmd

Answer 2

You can call instance method with classmethod when treating it like instance method and add class as instance. IMO it is really bad practice, but it solves your problem.

@classmethod
def new(cls):
    params = cls.get_params(cls)
    return cls(**params)

Answered By: Sajmon

Answer 3

You need to pass cls to an instance method from a class method as shown below to call the instance method from the class method:

class Person:
    def test1(self):
        print("Test1")
    
    @classmethod
    def test2(cls):
        cls.test1(cls) # Needed to pass "cls"

obj = Person()
obj.test2()

Output:

Test1

Answered By: Kai – Kazuya Ito

Python: How to call an instance method from a class method of the same class

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