Weak Reference Extension Module/Demo

Python 2.0b2 (#6, Sep 26 2000, 14:59:21) [MSC 32 bit (Intel)] on win32 Type "copyright", "credits" or "license" for more information. >>> import weakref >>> class Foo: pass ... >>> f = Foo () >>> wr = weakref.new (f) >>> wr A weak reference to an object of type 'instance'. >>> f1 = wr.newref () >>> f1 <__main__.Foo instance at 0081B63C> >>> f1 == f 1 >>> del f1 >>> del f >>> wr A stale weak reference. >>> type(wr.newref ()) (type, 'None')

1 # !/usr/local/bin/python 2 # 3 # Filename: circref_demo.py 4 # Copyright (c) Alex Shindich, 2000 5 # Author: Alex Shindich 6 # License: The author is making this software available on an "AS IS" basis. 7 # There are no warranties regarding this software. Use it at your own risk! 8 # Permission to use, copy, modify, and distribute this software and its 9 # documentation for any purpose and without fee is hereby granted without 10 # any restrictions. 11 # 12 # Purpose: This program demonstrates the circular reference problem. 13 14 class Subject: 15 """This class implements a very simple subject for observer pattern.""" 16 17 def __init__ (self): 18 self.__observers = [] 19 20 def addObserver (self, observer): 21 if observer.notify not in self.__observers: 22 self.__observers.append (observer.notify) 23 print "Added observer: %s" % str(observer) 24 25 def removeObserver (self, observer): 26 if observer.notify in self.__observers: 27 print "Removed observer: %s" % str(observer) 28 self.__observers.remove (observer.notify) 29 else: 30 print "Cannot remove an unknown observer: %s" % str(observer) 31 32 def notifyAll (self, msg): 33 if self.__observers: 34 for notify in self.__observers: 35 notify (msg) 36 else: 37 print "No observers to notify." 38 39 class Observer: 40 """This class implements a very simple observer for observer pattern.""" 41 42 def __init__(self, subject): 43 self.__subject = subject 44 self.__subject.addObserver (self) 45 46 def __del__ (self): 47 self.__subject.removeObserver (self) 48 print "Observer destroyed: %s" % str(self) 49 50 def notify (self, msg): 51 print "Notification received from the subject:\n\t'%s'" % msg 52 53 def main (): 54 # Create the subject 55 sub = Subject () 56 # Create an observer 57 obs = Observer (sub) 58 # Notify all the observers 59 sub.notifyAll ("Python rules!") 60 # Delete an observer 61 del obs 62 # Notify all the observers 63 sub.notifyAll ("There shouldn't be any observers at this point.") 64 65 66 if __name__ == '__main__': 67 main ()

Added observer: <__main__.Observer instance at 007F886C> Notification received from the subject: 'Python rules!' Notification received from the subject: 'There shouldn't be any observers at this point.'

   1    # !/usr/local/bin/python
   2    # 
   3    # Filename: circref_demo.py
   4    # Copyright (c) Alex Shindich, 2000
   5    # Author: Alex Shindich
   6    # License: The author is making this software available on an "AS IS" basis.
   7    #   There are no warranties regarding this software. Use it at your own risk!
   8    #   Permission to use, copy, modify, and distribute this software and its
   9    #   documentation for any purpose and without fee is hereby granted without
  10    #   any restrictions.
  11    #
  12    # Purpose: This program demonstrates the use of weakref module.
  13    
  14    try:
  15        import weakref
  16    except ImportError, ex:
  17        print ex
  18        print "Please make sure that weakref module is in the Python library path."
  19        import sys
  20        sys.exit (0)
  21       
  22    
  23    
  24    class Subject:
  25        """This class implements a very simple subject for observer pattern."""
  26    
  27        class Callback:
  28            """This class represents an observer  callback reference."""
  29    
  30            def __init__ (self, instanceMethod):
  31                self.__func  = instanceMethod.im_func
  32                self.__ob_self = weakref.new (instanceMethod.im_self)
  33    
  34            def __call__ (self, *args):
  35                ob_self = self.__ob_self.newref ()
  36                func = self.__func
  37                if ob_self and func:
  38                    apply (func, (ob_self,) + args)
  39                
  40        def __init__ (self):
  41            self.__observers = {}
  42    
  43        def addObserver (self, observer):
  44            # Create a weak reference that we will use for dictionary
  45            wr_observer = weakref.new (observer)
  46            # Check to see if the weak reference is in the dictionary already
  47            if not self.__observers.has_key (wr_observer):
  48                # Create a callback instance and add it to the dictionary
  49                cb = Subject.Callback (observer.notify)
  50                self.__observers[wr_observer]= cb
  51                print "Added observer: %s" % str(observer)
  52    
  53        def removeObserver (self, observer):
  54            # Create a weak reference that we will use for dictionary
  55            wr_observer = weakref.new (observer)
  56            # Check to see if the weak reference is in the dictionary
  57            if self.__observers.has_key (wr_observer):
  58                print "Removed observer: %s" % str(observer)
  59                # Delete the weak reference from the map
  60                del self.__observers[wr_observer]
  61            else:
  62                print "Cannot remove an unknown observer: %s" % str(observer)
  63    
  64        def notifyAll (self, msg):
  65            if self.__observers:
  66                for notify in self.__observers.values ():
  67                    notify (msg)
  68            else:
  69                print "No observers to notify."
  70    
  71    class Observer:
  72        """This class implements a very simple observer for observer pattern."""
  73    
  74        def __init__(self, subject):
  75            self.__subject = subject
  76            self.__subject.addObserver (self)
  77    
  78        def __del__ (self):
  79            self.__subject.removeObserver (self)
  80            print "Observer destroyed: %s" % str(self)
  81    
  82        def notify (self, msg):
  83            print "Notification received from the subject:\n\t'%s'" % msg
  84    
  85    def main ():
  86        # Create the subject
  87        sub = Subject ()
  88        # Create an observer
  89        obs = Observer (sub)
  90        # Notify all the observers
  91        sub.notifyAll ("Python rules!")
  92        # Delete an observer
  93        del obs
  94        # Notify all the observers
  95        sub.notifyAll ("There shouldn't be any observers at this point.")
  96    
  97    if __name__ == '__main__':
  98        main ()

Added observer: <__main__.Observer instance at 008254EC> Notification received from the subject: 'Python rules!' Removed observer: <__main__.Observer instance at 008254EC> Observer destroyed: <__main__.Observer instance at 008254EC> No observers to notify.