Can I create a singleton static class variable in python?

Question

I have the following scenario:

class A:
   b = 1
   pass

x = A()
y = A()

Can I change this class so that x.b = 2 is equivalent to A.b = 2, I mean, when a change the static variable for one instance it´s changed for all instances?

Edit: I want to be able to work with multiple different instances of this class.

Answer 1

You can, but it's kind of ugly:

class A:
    b = 1
    def __setattr__(self, name, value):
        if name == "b":
            setattr(A, "b", value)
        else:
            super().__setattr__(name, value)

This would work as expected now:

>>> a = A()
>>> a.b = 3
>>> A.b
3
>>> A.b = 5
>>> a.b
5

The real question is: Why would you want that?

---

If you're going to use this often, it might be nice to write a decorator for it:

def sharedclassvar(variable):
    def __setattr__(self, name, value):
        if name in self.__class__._sharedclassvars:
            setattr(self.__class__, name, value)
        elif hasattr(self.__class__, "__oldsetattr__"):
            self.__class__.__oldsetattr__(self, name, value)
        else:
            super().__setattr__(name, value)
    def decorator(cls):
        if not hasattr(cls, "_sharedclassvars"):
            cls._sharedclassvars = []
            if hasattr(cls, "__setattr__"):
                cls.__oldsetattr__ = getattr(cls, "__setattr__")
                cls.__setattr__ = __setattr__
        cls._sharedclassvars.append(variable)
        return cls
    return decorator

You can then define such a class like this:

@sharedclassvar("b")
class A:
    b = 1