"""
You need to use the Observer Pattern (http://en.wikipedia.org/wiki/Observer_pattern). 
In the following code, a person subscribes to receive updates from the global wealth entity. 
When there is a change to global wealth, this entity then alerts all 
its subscribers (observers) that a change happened. Person then updates itself.

I make use of properties in this example, but they are not necessary. 
A small warning: properties work only on new style classes, so 
the (object) after the class declarations are mandatory for this to work.
"""


class GlobalWealth(object):
    def __init__(self):
        self._global_wealth = 10.0
        self._observers = []

    @property
    def global_wealth(self):
        return self._global_wealth

    @global_wealth.setter
    def global_wealth(self, value):
        self._global_wealth = value
        for callback in self._observers:
            print('announcing change')
            callback(self._global_wealth)

    def bind_to(self, callback):
        print('bound')
        self._observers.append(callback)


class Person(object):
    def __init__(self, data):
        self.wealth = 1.0
        self.data = data
        self.data.bind_to(self.update_how_happy)
        self.happiness = self.wealth / self.data.global_wealth

    def update_how_happy(self, global_wealth):
        self.happiness = self.wealth / global_wealth


if __name__ == '__main__':
    data = GlobalWealth()
    p = Person(data)
    print(p.happiness)
    data.global_wealth = 1.0
    print(p.happiness)