1 class Singleton:
   2     """ A python singleton """
   3 
   4     class __impl:
   5         """ Implementation of the singleton interface """
   6 
   7         def spam(self):
   8             """ Test method, return singleton id """
   9             return id(self)
  10 
  11     # storage for the instance reference
  12     __instance = None
  13 
  14     def __init__(self):
  15         """ Create singleton instance """
  16         # Check whether we already have an instance
  17         if Singleton.__instance is None:
  18             # Create and remember instance
  19             Singleton.__instance = Singleton.__impl()
  20 
  21         # Store instance reference as the only member in the handle
  22         self.__dict__['_Singleton__instance'] = Singleton.__instance
  23 
  24     def __getattr__(self, attr):
  25         """ Delegate access to implementation """
  26         return getattr(self.__instance, attr)
  27 
  28     def __setattr__(self, attr, value):
  29         """ Delegate access to implementation """
  30         return setattr(self.__instance, attr, value)
  31 
  32 
  33 # Test it
  34 s1 = Singleton()
  35 print id(s1), s1.spam()
  36 
  37 s2 = Singleton()
  38 print id(s2), s2.spam()
  39 
  40 # Sample output, the second (inner) id is constant:
  41 # 8172684 8176268
  42 # 8168588 8176268