1、什么是单例模式     单例模式：基于某种方法实例化多次得到实例是同一个 2、为何用单例模式     当实例化多次得到的对象中存放的属性都一样的情况，应该将多个对象指向同一个内存，即同一个实例 3、如何用 ''' # 单例模式实现方式一： # import settings # # class Mysql: #     __instacne=None # #     def __init__(self,ip,port): #         self.ip=ip #         self.port=port # #     @classmethod #     def from_conf(cls): #         if cls.__instacne is None: #             cls.__instacne=cls(settings.IP,settings.PORT) #         return cls.__instacne # # obj=Mysql('1.1.1.10',3306) # # obj1=Mysql.from_conf() # obj2=Mysql.from_conf() # obj3=Mysql.from_conf() # # print(obj1) # print(obj2) # print(obj3) # # obj4=Mysql('10.10.10.11',3307) # 单例模式实现方式二： # import settings # def singleton(cls): #     cls.__instance=cls(settings.IP,settings.PORT) #     def wrapper(*args,**kwargs): #         if len(args) == 0 and len(kwargs) == 0: #             return cls.__instance #         return cls(*args,**kwargs) #     return wrapper # # @singleton #Mysql=singleton(Mysql) #Mysql=wrapper # class Mysql: #     def __init__(self,ip,port): #         self.ip=ip #         self.port=port # # # obj1=Mysql() #wrapper() # obj2=Mysql() #wrapper() # obj3=Mysql() #wrapper() # print(obj1 is obj2 is obj3) # print(obj1) # print(obj2) # print(obj3) # obj4=Mysql('1.1.1.4',3308) # print(obj4) # 单例模式实现方式三： import settings class Mymeta(type):     def __init__(self,class_name,class_bases,class_dic): #self=Mysql         super(Mymeta,self).__init__(class_name,class_bases,class_dic )         self.__instance=self.__new__(self) #造出一个Mysql的对象         self.__init__(self.__instance,settings.IP,settings.PORT) #从配置文件中加载配置完成Mysql对象的初始化         # print(self.__instance)         # print(self.__instance.__dict__)     def __call__(self, *args, **kwargs): #self=Mysql         if len(args) == 0 and len(kwargs) == 0:             return self.__instance         obj=self.__new__(self)         self.__init__(obj,*args,**kwargs)         return obj class Mysql(object,metaclass=Mymeta): #Mysql=Mymeta(...)     def __init__(self,ip,port):         self.ip=ip         self.port=port obj1=Mysql() obj2=Mysql() obj3=Mysql() obj4=Mysql('10.10.10.11',3308) print(obj1) print(obj2) print(obj3) print(obj4)