# 单例模式：# 实现方式一：class Mysql: __instance=None # __instance=obj def __init__(self): self.host = '127.0.0.1' self.port = 3306 @classmethod def singleton(cls): if not cls.__instance: # 空值是真，执行下面代码 ；有值是假直接返回 obj = Mysql() cls.__instance = obj # __instance=obj return cls.__instance def conn(self): passobj1 = Mysql.singleton()obj2 = Mysql.singleton()obj3 = Mysql.singleton()print(obj1 is obj2) # Trueprint(obj1 is obj3) # Trueprint(obj1, obj2, obj3)'''执行结果：<__main__.Mysql object at 0x000001F87E050F98> <__main__.Mysql object at 0x000001F87E050F98> <__main__.Mysql object at 0x000001F87E050F98>'''# 实现方式二：元类的方式class Mymeta(type): __instance = None def __init__(self, class_name, class_bases, class_dic): if not class_name.istitle(): # class_name 首字母不是大写就报类型错误 raise TypeError('类型错误') if '__doc__' not in class_dic or not class_dic['__doc__'].strip(): raise TypeError('必需要有注释且不能为空') super(Mymeta, self).__init__(class_name, class_bases, class_dic) def __call__(self, *args, **kwargs): # obj = Chinese('kingforn',age=18) if not self.__instance: obj = object.__new__(self) self.__init__(obj, *args, **kwargs) self.__instance = obj return self.__instanceclass Mysql(object, metaclass=Mymeta): '''元类控制实例化''' def __init__(self): self.host = '127.0.0.1' self.port = 3306 def conn(self): passobj1 = Mysql()obj2 = Mysql()obj3 = Mysql()print(obj1 is obj2) # Trueprint(obj1 is obj3) # Trueprint(obj1, obj2, obj3)'''结果：<__main__.Mysql object at 0x0000029605CB6C18> <__main__.Mysql object at 0x0000029605CB6C18> <__main__.Mysql object at 0x0000029605CB6C18>'''
来源：https://www.cnblogs.com/kingforn/p/11360172.html