#!/usr/bin/env python
# -*- coding: utf-8 -*-"""
http://code.activestate.com/recipes/131499-observer-pattern/*TL;DR80
Maintains a list of dependents and notifies them of any state changes.
"""from __future__ import print_functionclass Subject(object):def __init__(self):self._observers = []def attach(self, observer):if observer not in self._observers:self._observers.append(observer)def detach(self, observer):try:self._observers.remove(observer)except ValueError:passdef 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 = nameself._data = 0@propertydef data(self):return self._data@data.setterdef data(self, value):self._data = valueself.notify()class HexViewer:def update(self, subject):print(u'HexViewer: Subject %s has data 0x%x' %(subject.name, subject.data))class DecimalViewer:def update(self, subject):print(u'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(u"Setting Data 1 = 10")data1.data = 10print(u"Setting Data 2 = 15")data2.data = 15print(u"Setting Data 1 = 3")data1.data = 3print(u"Setting Data 2 = 5")data2.data = 5print(u"Detach HexViewer from data1 and data2.")data1.detach(view2)data2.detach(view2)print(u"Setting Data 1 = 10")data1.data = 10print(u"Setting Data 2 = 15")data2.data = 15if __name__ == '__main__':main()### OUTPUT ###
# Setting Data 1 = 10
# DecimalViewer: Subject Data 1 has data 10
# HexViewer: Subject Data 1 has data 0xa
# Setting Data 2 = 15
# HexViewer: Subject Data 2 has data 0xf
# DecimalViewer: Subject Data 2 has data 15
# Setting Data 1 = 3
# DecimalViewer: Subject Data 1 has data 3
# HexViewer: Subject Data 1 has data 0x3
# Setting Data 2 = 5
# HexViewer: Subject Data 2 has data 0x5
# DecimalViewer: Subject Data 2 has data 5
# Detach HexViewer from data1 and data2.
# Setting Data 1 = 10
# DecimalViewer: Subject Data 1 has data 10
# Setting Data 2 = 15
# DecimalViewer: Subject Data 2 has data 15