Object-oriented reflection, built-in methods, metaclasses, attribute search operations

1. Control in the metaclass to change the data attributes of the custom class to uppercase

class Mymeta(type):
    def __new__(cls,x,y,z):#其中x为类名，y为基类，z为名称空间
        class_dic = {}
        for k,v in z.items():
            if not callable(v) and not k.startswith('__'):
                class_dic[k.upper()] = v
            else:
                class_dic[k] = v
        return type.__new__(cls,x,y,class_dic)


class People(metaclass = Mymeta):
    country = 'China'
    tag = '龙的传人'
    def __init__(self,name,age):
        self.name = name
        self.age = age

​    def say(self):
​        print('<%s:%s>'%(self.name,self.age))

print(People.__dict__)

2. The __init__ method is not required to control the custom class in the metaclass

1) Metaclasses help them to create objects and initialize operations;

2) The parameter must be in the form of keyword during instantiation, otherwise an exception is thrown TypeError: must use keyword argument

3) The key is the attribute of the object generated by the user-defined class, and all attributes become uppercase

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

class People(metaclass = Mymeta):

​    country = 'China'
​    tag = '龙的传人'
​    def singing(self):
​        print('[{}]正在唱: 万里长城永不倒。。。'.format(self.NAME))

tank = People(name = 'tank', age = 18)
print(tank.__dict__)
tank.singing()

3. In the metaclass, all attributes related to the objects generated by the custom class are hidden attributes.

class Mymeta(type):
    def __call__(self,*args,**kwargs):
        people_obj = object.__new__(self)
        self.__init__(people_obj,*args,**kwargs)
        people_obj.__dict__={
        '_%s__%s'%(self.__name__,k):v for k,v in people_obj.__dict__.items()
        }
        return people_obj

class People(metaclass = Mymeta):
    country = 'China'
    tag = '龙的传人'
    def __init__(self,name,age):
        self.name = name
        self.age = age
    def singing(self):
        print('[{}]正在唱：万里长城永不倒。。。'.format(self._People__name))

tank = People('tank',84)
tank.singing()
print(tank.tag)

4. Implement the singleton pattern based on metaclasses

import settings

class Mymeta(type):
    def __init__(self,name,bases,dic):
        self.__instance = object.__new__(self)
        self.__init__(self.__instance,settings.HOST,settings.PORT)
        super().__init__(name,bases,dic)

​    def __call__(self,*args,**kwargs):
​        if args or kwargs:
​            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()
obj2 = Mysql()
obj3 = Mysql()
print(obj1 is obj2 is obj3)