event.py

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)

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)