自定义元类控制类的实例化行为的应用

# 单例模式:对一个类多次实例化，但只实例化出同一个对象
class Mymeta(type):
    def __init__(self,class_name,class_bases,class_dic):
        if not class_name.istitle():
            raise TypeError('类名的首字母必须大写！')
        if '__doc__' not in class_dic or not class_dic['__doc__'].strip():
            raise TypeError('必须有注释，且注释不能为空！')
        super(Mymeta,self).__init__(class_name,class_bases,class_dic)
        self.__instance = None

    def __call__(self, *args, **kwargs):
        if not self.__instance:
            obj = object.__new__(self)
            self.__init__(obj,*args, **kwargs)
            self.__instance=obj
        return self.__instance

class Mysql(object,metaclass=Mymeta):
    '''
    MYSQL
    '''
    def __init__(self):
        self.host='127.0.0.1'
        self.port=3306

obj1 = Mysql()
obj2 = Mysql()
obj3 = Mysql()

print(obj1)
print(obj2)
print(obj3)