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

class Subject(object):

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

@property

def data(self):

return self._data

@data.setter

def data(self, value):

self._data = value

self.notify()

class HexViewer:

def update(self, subject):

print('HexViewer: Subject %s has data 0x%x' %

(subject.name, subject.data))

class DecimalViewer:

def update(self, subject):

print('DecimalViewer: Subject %s has data %d' %

(subject.name, subject.data))

# 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.data = 10

print("Setting Data 2 = 15")

data2.data = 15

print("Setting Data 1 = 3")

data1.data = 3

print("Setting Data 2 = 5")

data2.data = 5

print("Detach HexViewer from data1 and data2.")

data1.detach(view2)

data2.detach(view2)

print("Setting Data 1 = 10")

data1.data = 10

print("Setting Data 2 = 15")

data2.data = 15

if __name__ == '__main__':

main()