路飞学城Python-Day20(元类的练习题)

练习一：在元类中控制把自定义类的数据属性都变成大写

class MyDef(type):
    def __new__(cls, class_name, class_attr, class_dic):
        upper_data = {}
        for k,v in class_dic.items():
            if not callable(v) and not k.startswith('__'):
                upper_data[k.upper()] = v
            else:
                upper_data[k] = v
        return type.__new__(cls, class_name, class_attr, upper_data)

class People(metaclass=MyDef):
    time = 'now'
    work = 'weekends'

print(People.__dict__)

练习二：在元类中控制自定义的类无需__init__方法

 

1.元类帮其完成创建对象，以及初始化操作；

2.要求实例化时传参必须为关键字形式，否则抛出异常TypeError: must use keyword argument

3.key作为用户自定义类产生对象的属性，且所有属性变成大写

class MyDef(type):
    def __call__(self, *args, **kwargs):
        if args:
            raise TypeError(' must use keyword argument')
        obj = object.__new__(self)
        for k,v in kwargs.items():
            obj.__dict__[k.upper()] = v
        return obj

class People(metaclass=MyDef):
    tag = 'China'
    some = 'None'

p1 = People(name = 'panda')
print(p1.__dict__)

练习三：在元类中控制自定义的类产生的对象相关的属性全部为隐藏属性

# 在元类中控制自定义的类产生的对象相关的属性全部为隐藏属性
class Mymeta(type):
    def __init__(self, class_name, class_bases, attr):
        super(Mymeta, self).__init__(class_name, class_bases, attr)

    def __call__(self, *args, **kwargs):
        obj = self.__new__(self)
        self.__init__(obj, *args, **kwargs )
        obj.__dict__ = {'_%s__%s' % (self.__name__, k): v for k, v in obj.__dict__.items()}
        return obj
class P(metaclass=Mymeta):
    def __init__(self, name, age):
        self.name = name
        self.age = age
p1 = P('','')
print(p1._P__name)

 

练习四：基于元类实现单例模式