'''http://code.activestate.com/recipes/131499-observer-pattern/'''

class Subject:
    def __init__(self):
        self._observers = []

    def attach(self, observer):
        if not observer in self._observers:
            self._observers.append(observer)

    def detach(self, observer):
        try:
            self._observers.remove(observer)
        except ValueError:
            pass

    def notify(self, modifier=None):
        for observer in self._observers:
            if modifier != observer:
                observer.update(self)


# Example usage
class Data(Subject):
    def __init__(self, name=''):
        Subject.__init__(self)
        self.name = name
        self.data = 0

    def setData(self, data):
        self.data = data
        self.notify()

    def getData(self):
        return self.data


class HexViewer:
    def update(self, subject):
        print('HexViewer: Subject %s has data 0x%x' % (subject.name, subject.getData()))


class DecimalViewer:
    def update(self, subject):
        print('DecimalViewer: Subject %s has data %d' % (subject.name, subject.getData()))


# Example usage...
def main():
    data1 = Data('Data 1')
    data2 = Data('Data 2')
    view1 = DecimalViewer()
    view2 = HexViewer()
    data1.attach(view1)
    data1.attach(view2)
    data2.attach(view2)
    data2.attach(view1)

    print("Setting Data 1 = 10")
    data1.setData(10)
    print("Setting Data 2 = 15")
    data2.setData(15)
    print("Setting Data 1 = 3")
    data1.setData(3)
    print("Setting Data 2 = 5")
    data2.setData(5)
    print("Detach HexViewer from data1 and data2.")
    data1.detach(view2)
    data2.detach(view2)
    print("Setting Data 1 = 10")
    data1.setData(10)
    print("Setting Data 2 = 15")
    data2.setData(15)

if __name__ == '__main__':
    main()