1、什麽是單例模式
    單例模式：基於某種方法實例化多次得到實例是同一個
2、為何用單例模式
    當實例化多次得到的對象中存放的屬性都一樣的情況，應該將多個對象指向同一個內存，即同一個實例
3、如何用
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# 單例模式實現方式一：
# 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)


# 單例模式實現方式二：
# 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)




# 單例模式實現方式三：
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) #造出一個Mysql的對象
        self.__init__(self.__instance,settings.IP,settings.PORT) #從配置文件中加載配置完成Mysql對象的初始化

        # 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)

單例模式實現的三種方式