自定义元类控制类的实例化行为（二）

# 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):
        pass


obj1 = Mysql.singleton()
obj2 = Mysql.singleton()
obj3 = Mysql.singleton()
print(obj1 is obj2)             # True
print(obj1 is obj3)             # True
print(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.__instance


class 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):
        pass


obj1 = Mysql()
obj2 = Mysql()
obj3 = Mysql()
print(obj1 is obj2)             # True
print(obj1 is obj3)             # True
print(obj1, obj2, obj3)
'''结果：
<__main__.Mysql object at 0x0000029605CB6C18> <__main__.Mysql object at 0x0000029605CB6C18> <__main__.Mysql object at 0x0000029605CB6C18>
'''