python单例设计模式

 class Dog(object):

     __instance = None

     def __init__(self):

         pass

     def __new__(cls):

         if not cls.__instance:

             cls.__instance = object.__new__(cls)

         return cls.__instance

 d = Dog()

 dd = Dog()

 #d和dd的id相同

 print(id(d))

 print(id(dd))