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))