Python 单例模式

class Dog(object):

    __instance = None   # 设置一个变量来判断储存构造出来得例子

    def __new__(cls):
        if cls.__instance == None:
            cls.__instance = object.__new__(cls)
            return cls.__instance
        else:
            # return 上一次创建得对象得引用
            return cls.__instance

a = Dog()
print(id(a))
b = Dog()
print(id(b))