元类作业

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 Mymetaclass(type):
    def __new__(cls,name,bases,attrs):
        update_attrs={}
        for k,v in attrs.items():
            if not callable(v) and not k.startswith('__'):
                update_attrs[k.upper()]=v
            else:
                update_attrs[k]=v
        return type.__new__(cls,name,bases,update_attrs)

class Chinese(metaclass=Mymetaclass):
    country='China'
    tag='Legend of the Dragon' 
    def walk(self):
        print('%s is walking' %self.name)

print(Chinese.__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 Mymeta(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 a(metaclass=Mymeta):
    tag= "legond of Dragon"
    def walk(self):
        print("%s is walking "%self.tag)


p = a(name='egon',age=18,sex='male')
print(p.__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

class mt(type):
    def __init__(self,classname,bases,class_dic):
        print(self)#<class '__main__.Foo'>
        print(classname)#Foo
        print(bases)#(<class 'object'>,)
        print(class_dic)#{'__module__': '__main__', '__qualname__': 'Foo', '__init__': <function Foo.__init__ at 0x00000000024AAC80>}
        super().__init__(classname,bases,class_dic)

    def __call__(self, *args, **kwargs):
        print(self)#<class '__main__.Foo'>
        print(args)#('egon', 18, 'man')
        print(kwargs)#{}
        obj = object.__new__(self)
        self.__init__(self,*args,**kwargs)
        for k,v in self.__dict__.items():
            obj.__dict__["_%s__%s"%(self.__name__,k)] = v
        return obj


class Foo(object,metaclass=mt):
    def __init__(self,name,age,sex):
        self.name = name
        self.age = age
        self.sex = sex


a = Foo("egon",18,"man")
print(a.__dict__)
"""
'_Foo____module__': '__main__', '_Foo____init__': <function Foo.__init__ at 0x000000000251AD08>, 
'_Foo____dict__': <attribute '__dict__' of 'Foo' objects>, 
'_Foo____weakref__': <attribute '__weakref__' of 'Foo' objects>, 
'_Foo____doc__': None, '_Foo__name': 'egon', '_Foo__age': 18, '_Foo__sex': 'man'}
"""

练习四：基于元类实现单例模式

"""
练习四：基于元类实现单例模式
"""
class mt(type):
    instance = None
    def __call__(self, *args, **kwargs):
        if not mt.instance:
            mt.instance = self.__new__(self)#生成一个空对象
            mt.instance.__init__(*args,**kwargs)#初始化对象
        return mt.instance #返回对象


class Foo(object,metaclass=mt):
    def __init__(self,name,age):
        self.name = name
        self.age = age
    def walk(self):
        print("%s is walking"%self.name)


p1 = Foo("egon",18)
p2 = Foo("egon",18)

print(p1)
print(p2)