How to trigger function on value change?

Question

I realise this question has to do with event-handling and i've read about Python event-handler a dispatchers, so either it did not answer my question or i completely missed out the information.

I want method m() of object A to be triggered whenever value v is changing:

For instance (assuming money makes happy):

global_wealth = 0  class Person()     def __init__(self):         self.wealth = 0         global global_wealth         # here is where attribute should be         # bound to changes in 'global_wealth'         self.happiness = bind_to(global_wealth, how_happy)      def how_happy(self, global_wealth):         return self.wealth / global_wealth 

So whenever the global_wealth value is changed, all instances of the class Person should change their happiness value accordingly.

NB: I had to edit the question since the first version seemed to suggest i needed getter and setter methods. Sorry for the confusion.

Answer 1

You need to use the 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) 

Answer 2

You can use properties if you want to execute code when attributes are changed. Be wary that big side-effects or significant overhead occurring when an attribute is changed is a little bit surprising to anyone using your API, so in some cases you probably want to avoid it by using methods instead.

class A(object):      def m(self, p_value):          print p_value      @property     def p(self):         return self._p       @p.setter     def p(self, value):         self._p = value         self.m(value)