Set attribute of subclass in method of super class

Question

I have the following scenario:

class Baseclass(object):
    extra_fields = []
    @classmethod
    def extend(cls, key):
        cls.extra_fields.append(key)

class A(Baseclass):
    pass

class B(Baseclass):
    pass

A.extend("foo")

Now, extend of Baseclass will be called, setting Baseclass.extra_fields to ["foo"] . Then, A.extra_fields will be ["foo"] , however so will B.extra_fields .

Is there a way in extend to only modify only the subclass on which it was invoked (without defining extend on all subclasses, as those may not be known in advance)?

Answer 1

class Baseclass(object):    
    @classmethod
    def extend(cls, key):
        if not 'extra_fields' in cls.__dict__:
            cls.extra_fields=[]
        cls.extra_fields.append(key)   

class A(Baseclass):
    pass

class B(Baseclass):
    pass

A.extend("foo")

Answer 2

How about defining own extra_fields for each subclass?

class Baseclass(object):
    @classmethod
    def extend(cls, key):
        cls.extra_fields.append(key)

class A(Baseclass):
    extra_fields = []

class B(Baseclass):
    extra_fields = []

A.extend("foo")

(You can automate that with some metaclasses magic as well.)