python-patterns/patterns/behavioral/observer.py at master · Androidown/python-patterns · GitHub

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#!/usr/bin/env python
# -*- coding: utf-8 -*-

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

*TL;DR
Maintains a list of dependents and notifies them of any state changes.

*Examples in Python ecosystem:
Django Signals: https://docs.djangoproject.com/en/2.1/topics/signals/
Flask Signals: http://flask.pocoo.org/docs/1.0/signals/
"""

from __future__ importprint_function


classSubject(object):
def__init__(self):
self._observers= []

defattach(self, observer):
ifobservernotinself._observers:
self._observers.append(observer)

defdetach(self, observer):
try:
self._observers.remove(observer)
exceptValueError:
pass

defnotify(self, modifier=None):
forobserverinself._observers:
ifmodifier!=observer:
observer.update(self)


# Example usage
classData(Subject):
def__init__(self, name=''):
Subject.__init__(self)
self.name=name
self._data=0

@property
defdata(self):
returnself._data

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


classHexViewer:
defupdate(self, subject):
print(u'HexViewer: Subject %s has data 0x%x'% (subject.name, subject.data))


classDecimalViewer:
defupdate(self, subject):
print(u'DecimalViewer: Subject %s has data %d'% (subject.name, subject.data))


# Example usage...
defmain():
"""
 >>> data1 = Data('Data 1')
 >>> data2 = Data('Data 2')
 >>> view1 = DecimalViewer()
 >>> view2 = HexViewer()
 >>> data1.attach(view1)
 >>> data1.attach(view2)
 >>> data2.attach(view2)
 >>> data2.attach(view1)

 >>> data1.data = 10
 DecimalViewer: Subject Data 1 has data 10
 HexViewer: Subject Data 1 has data 0xa

 >>> data2.data = 15
 HexViewer: Subject Data 2 has data 0xf
 DecimalViewer: Subject Data 2 has data 15

 >>> data1.data = 3
 DecimalViewer: Subject Data 1 has data 3
 HexViewer: Subject Data 1 has data 0x3

 >>> data2.data = 5
 HexViewer: Subject Data 2 has data 0x5
 DecimalViewer: Subject Data 2 has data 5

 # Detach HexViewer from data1 and data2
 >>> data1.detach(view2)
 >>> data2.detach(view2)

 >>> data1.data = 10
 DecimalViewer: Subject Data 1 has data 10

 >>> data2.data = 15
 DecimalViewer: Subject Data 2 has data 15
 """


if__name__=="__main__":
importdoctest
doctest.testmod()