class Singleton:
""" A python singleton """
class __impl:
""" Implementation of the singleton interface """
def spam(self):
""" Test method, return singleton id """
return id(self)
# storage for the instance reference
__instance = None
def __init__(self):
""" Create singleton instance """
# Check whether we already have an instance
if Singleton.__instance is None:
# Create and remember instance
Singleton.__instance = Singleton.__impl()
# Store instance reference as the only member in the handle
self.__dict__['_Singleton__instance'] = Singleton.__instance
def __getattr__(self, attr):
""" Delegate access to implementation """
return getattr(self.__instance, attr)
def __setattr__(self, attr, value):
""" Delegate access to implementation """
return setattr(self.__instance, attr, value)
# Test it
s1 = Singleton()
print id(s1), s1.spam()
s2 = Singleton()
print id(s2), s2.spam()
# Sample output, the second (inner) id is constant:
# 8172684 8176268
# 8168588 8176268
