Question

I’d like to set a class property that can be shared by all instances of the class or its subclasses. It should be possible for any instance to also set the property.

I tried the following:

class A:
    x = 1
    @classmethod
    def setX(cls, val):
        if cls.__bases__:
            cls = cls.__bases__[-1]
        cls.x = val

This seems to work fine in case of single inheritance. But if I use multiple inheritance, depending on the order of inheritance, it either works or doesn’t (i.e., class A is not always the last of bases).

Any ideas for a robust implementation?

Answer 1

The right way, IMO, is to go with a class property. Since properties are tied to an object’s classes __dict__, as opposed to the object’s very own __dict__, and since your object happens to be a class, you must attach it to the classes class, its metaclass:

class A(object):

    _x = None

    class __metaclass__(type):

        @property
        def x(cls):
            return A._x

        @x.setter
        def x(cls, val):
            A._x = val


class B(A):
    pass

A.x = 'jim'
B.x = 'joe'

print A.x, B.x

Result:

joe joe

Also, your classes must be new style classes, i.e. they must inherit from object in Python 2.x. And metaclasses are defined differently in Python 3.x:

class MetaA(type):
    """ like ___metaclass__ above """

class A(metaclass=MetaA):

    _x = None

...