单例的三种实现方式

一：类方法实现单例

class Mysql:
    __instance = None
    def __init__(self,host,port):
        self.host = host
        self.port = port  q
    @classmethod
    def singleton(cls):
        if not cls.__instance:
            cls.__instance = cls(setting.host,setting.port)
        return cls.__instance
obj1 = Mysql.singleton()
obj2 = Mysql.singleton()
print(obj1)
print(obj2)

二：自定制元类方法实现单例

import setting
class Mymeta(type):
    def __init__(self,class_name,base,dic):
        super().__init__(class_name, base, dic)#初始化一个类（mysql）
        self.__instance = object.__new__(self)#利用mysql类的基类__new__方法造出一个mysql类的对象
        self.__init__(self.__instance,setting.host,setting.port)#调用类的__init__方法实例化出这个对象
    def __call__(self, *args, **kwargs):
        #如果对元类的实例化对象mysql类加括号调用则自动触发，元类的对象类mysql会作为第一个参数自动传入
        #相当于mysql.__call__()
        if args or kwargs:
            obj = object.__new__(self)
            self.__init__(obj,*args,**kwargs)
            return obj
        return self.__instance
class Mysql(metaclass=Mymeta):#定义类mysql（实际就是（调用元类的_init__方法））
    def __init__(self,host,port):
        self.host = host
        self.port = port
obj1 = Mysql()#加括号调用，实例化生成对象（调用类的__init__方法）
obj2 = Mysql()
print(obj1)
print(obj2)
obj3 = Mysql("129.0.0.1",8080)
obj4 = Mysql("129.0.0.1",8080)
print(obj3)
print(obj4)

三：自定制元类方法实现单例

def sigleton(cls):
    __instance = cls(setting.host,setting.port)
    def wrapper(*args,**kwargs):
        if args or kwargs:
            # obj = cls(*args, **kwargs)
            obj = object.__new__(cls)
            cls.__init__(obj,*args, **kwargs)
            return obj
        return __instance
    return wrapper
@sigleton
class Mysql:
    def __init__(self,host,port):
        self.host = host
        self.port = port

附：setting.py

port = 8888
host = "127.0.0.1"