Detailed introduction to attribute descriptors in python (code example)

This article brings you a detailed introduction (code example) about attribute descriptors in python. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

As a novice, I am constantly reading things and learning knowledge every day. Today I would like to introduce a good thing to you-Attribute descriptor

What is an attribute descriptor?

In fact, any magic function among set__, __get__, __delete implemented in a class is an attribute descriptor.

Next we define an attribute descriptor:

class IntegerField:
    def __get__(self, instance, owner):
        pass
    def __set__(self, instance, value):
        pass
    def __delete__(self, instance):
        pass
class User:
    high= IntField()

__get__: When we call this attribute with a class or instance, the result of the __get__ function will be returned.
__set__: Python calls this function when we use Instance to set the attribute value. There are no restrictions on classes.
__delete__: Python will call this function when we try to delete the attribute using an instance. There are no restrictions on classes.
How to use this thing? Next, I will modify the above code for everyone

class IntegerField:
    def __get__(self, instance, owner):
        return self.value
    def __set__(self, instance, value):
        if not isinstance(value,numbers.Integral):
            raise ValueError("请输入一个整数")
        self.value=value
        
    def __delete__(self, instance):
        pass
class User:
    high=IntegerField() 
#验证代码
if __name__ == '__main__':
    user=User()
    user.high='175'    #报错,ValueError:请输入一个整数
    User.high=175    #正确执行，不报错

so that we can use attribute descriptors to attach certain logic to the attributes.
In fact, under the attribute descriptor, it is also divided into
1. Data descriptor: implements __set__, __get__

if __name__ == '__main__':
    user=User()
    user.high=175
    print(user.__dict__)    #high是不放入__dict__中的，优先查找数据描述符中的值
    user.__dict__["high"]="abc"    #这样赋值时可以的,并且可以放入__dict__中
    print(user.high)    #会报错，因为在调用__get__方法时并没有value属性

2. Non-data: implements __get__ but does not implement __set__

class NonField:
    def __init__(self, high=170):
        self.value = high

    def __get__(self, instance, owner):
        return self.value


class User:
    high = NonField()


if __name__ == '__main__':
    user = User()
    user.high = '175'    #会放入user.__dict__中
    print(user.__dict__)

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