Share class attributes among several classes in Python

Question

I have a class that contains class attributes. I need that they are shared as class attributes by a group of other classes, so that if any of them modifies those attributes, they are modified for all of them. I naively tried it using inheritance, but it is not working like that: each class behaves as if it would have their own "copy" of those class attributes.

Files:

# baseClass.py 
class BaseClass(object):
    iden = 0


# child1.py 
from baseClass import BaseClass

class Child1(BaseClass):
    @classmethod
    def getId(cls):
        return cls.iden


# child2.py
from baseClass import BaseClass

class Child2(BaseClass):
    @classmethod
    def getId(cls):
        return cls.iden

The file I am using to test this is:

from child1 import Child1
from child2 import Child2

print(Child1.getId()) 
Child1.iden = 5
print(Child1.getId())
print(Child2.getId())

The output I am seeing:

0
5
0

How should I do this, so that Child1, Child2 and BaseClass share the iden class attribute?

Answer 1

You can do this by adding an iden property to the metaclass of BaseClass:

class BaseType(type):
    _iden = 0
    def _get_iden(cls):
        return BaseType._iden
    def _set_iden(cls, value):
        BaseType._iden = value
    iden = property(_get_iden, _set_iden)

class BaseClass(object):
    __metaclass__ = BaseType

class Child1(BaseClass):
    pass

class Child2(BaseClass):
    pass

print Child1.iden
Child1.iden = 5
print Child1.iden
print Child2.iden

Output:

0
5
5