observer.py

#!/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_function


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


if __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