class Desc:
    def __set_name__(self, owner, name):
        self.name = '_' + name
    def __get__(self, obj, t=None):
        return getattr(obj, self.name, 0)
    def __set__(self, obj, v):
        setattr(obj, self.name, v * 2)

class A:
    x = Desc()

a = A(); a.x = 5
print(a.x)