python class decorator

Question

提问在注释里

def object_counter(class_):
    class_.__getattr__orig = class_.__getattribute__#class_.__getattribute__是一个method, 这里是赋值给变量的意思？

    def new_getattr(self, name):
        if name == 'mileage':
            print('We noticed that the mileage attribute was read')
        return class_.__getattr__orig(self, name)#能否解释一下这个return的意义
    
    class_.__getattribute__ = new_getattr#这里也看不懂
    
    return class_#这里的class_到底算变量，还是类
@object_counter
class Car:
    def __init__(self, VIN):
        self.mileage = 0
        self.VIN = VIN

car = Car('ABC123')
print('The mileage is', car.mileage)
print('The VIN is', car.VIN)

Answer 1

def object_counter(class_):
    class_.__getattr__orig = class_.__getattribute__#这个动态给参数class_添加新的方法__getattr__orig，指向class_对象元素的__getattribute__获取属性的方法
 
    def new_getattr(self, name):
        if name == 'mileage':#这里做属性拦截，访问某个属性时，name变量存储要访问的属性名，如果访问属性名mileage，则打印下面的信息
            print('We noticed that the mileage attribute was read')
        if name=="xxx":#这属性没有定义，可以直接返回一个新值，不能调用下面的代码，会出错
            return "xxx"
        return class_.__getattr__orig(self, name)#这里调用原始的对象__getattribute__方法获取属性值后作为函数返回值
    
    class_.__getattribute__ = new_getattr#重新定义对象的__getattribute__指向新的方法new_getattr    
    
    print(type(class_))
    return class_#这里的class_为动态新增了__getattr__orig方法的Car类
@object_counter
class Car:
    def __init__(self, VIN):
        self.mileage = 0
        self.VIN = VIN

car = Car('ABC123')
print('The mileage is', car.mileage)
print('The VIN is', car.VIN)

print('The VIN is', car.xxx)#未定义的属性