作业20200415

1、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' #龙的传人
    def walk(self):
        print('%s is walking' %self.name)


print(Chinese.__dict__)

2、在元类中控制自定义的类无需__init__方法

class Mymetaclass(type):
    def __call__(self, *args, **kwargs):
        if args:
            raise TypeError('must use keyword argument for key function')
        obj = object.__new__(self) #创建对象，self为类Foo

        for k,v in kwargs.items():
            obj.__dict__[k.upper()]=v
        return obj

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


p=Chinese(name='lili',age=18,sex='male')
print(p.__dict__)

3、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,class_dic):
        #控制类Foo的创建
        super(Mymeta,self).__init__(class_name,class_bases,class_dic)

    def __call__(self, *args, **kwargs):
        #控制Foo的调用过程，即Foo对象的产生过程
        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 Foo(object,metaclass=Mymeta):  # Foo=Mymeta(...)
    def __init__(self, name, age,sex):
        self.name=name
        self.age=age
        self.sex=sex


obj=Foo('lili',18,'male')
print(obj.__dict__)

4、基于元类实现单例模式

#settings.py文件内容如下
HOST='1.1.1.1'
PORT=3306

#方式一:定义一个类方法实现单例模式
import settings

class Mysql:
    __instance=None
    def __init__(self,host,port):
        self.host=host
        self.port=port

    @classmethod
    def singleton(cls):
        if not cls.__instance:
            cls.__instance=cls(settings.HOST,settings.PORT)
        return cls.__instance

obj1=Mysql('1.1.1.2',3306)
obj2=Mysql('1.1.1.3',3307)
print(obj1 is obj2) #False

obj3=Mysql.singleton()
obj4=Mysql.singleton()
print(obj3 is obj4) #True


#方式二：定制元类实现单例模式
import settings

class Mymeta(type):
    def __init__(self,name,bases,dic): #定义类Mysql时就触发
        # 事先先从配置文件中取配置来造一个Mysql的实例出来
        self.__instance = object.__new__(self)  # 产生对象
        self.__init__(self.__instance, settings.HOST, settings.PORT)  # 初始化对象
        # 上述两步可以合成下面一步
        # self.__instance=super().__call__(*args,**kwargs)
        super().__init__(name,bases,dic)

    def __call__(self, *args, **kwargs): #Mysql(...)时触发
        if args or kwargs: # args或kwargs内有值
            obj=object.__new__(self)
            self.__init__(obj,*args,**kwargs)
            return obj
        return self.__instance

class Mysql(metaclass=Mymeta):
    def __init__(self,host,port):
        self.host=host
        self.port=port

obj1=Mysql() # 没有传值则默认从配置文件中读配置来实例化，所有的实例应该指向一个内存地址
obj2=Mysql()
obj3=Mysql()
print(obj1 is obj2 is obj3)
obj4=Mysql('1.1.1.4',3307)