class Observer(object):
    def __init__(self):
        super().__setattr__('_subscribers', {})

    def subscribe(self, name, cb):
        if name not in self._subscribers:
            self._subscribers[name] = []
        self._subscribers[name].append(cb)

    def __setattr__(self, name, val):
        if name in self._subscribers:
            for cb in self._subscribers[name]:
                cb(val)
        super().__setattr__(name, val)