Simple demonstration of observer pattern

observer.py

"""
This is a simple demonstration of observer pattern
"""
__author__ = 'AhmetCanSolak'


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

    def add_observer(self, observer):
        if observer not in Subject.observers:
            Subject.observers.append(observer)

    def remove_observer(self, observer):
        try:
            Subject.observers.remove(observer)
        except ValueError:
            return None

    def notify(self, source=None):
        print("notify is called")
        for observer in Subject.observers:
            if observer != source:
                print("update called")
                observer.update() # change to call update method of viewer/container


class Image(Subject):
    def __init__(self, name=''):
        Subject.__init__(self)
        self.name = name
        self._data = [1]

    @property
    def data(self):
        return self._data

    @data.setter
    def data(self, value):
        self._data = value
        self.notify(self)

    def update(self):
        print(self.name, self._data)


# Initialize
img1 = Image("im1")
img2 = Image("im2")
img3 = Image("im3")
img4 = Image("im4")

sub = Subject()
sub.add_observer(img1)
sub.add_observer(img2)
sub.add_observer(img3)
sub.add_observer(img4)

# Change value of one to verify demo is working
img2.data = [4]

# keep changing data of one and observers will notify on each change
while input() != 'c':
    img2.data = [img2.data[0] + 1]