描述器

https://docs.python.org/zh-cn/3/reference/datamodel.html#implementing-descriptors

一个简单的描述器

class CharFiled:

    def __init__(self, max_length=20):
        self.max_length = max_length

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

    def __set__(self, instance, value):
        if isinstance(self.value, str):
            if len(value) <= self.max_length:
                self.value = value
            else:
                raise ValueError('字符串长度在{}以内'.format(self.max_length))
        else:
            raise TypeError('need a str')

    def __delete__(self, instance):
        self.value = None


class UserModel:
    """
    模型类
    """
    name = CharFiled(max_length=20)
    pwd = CharFiled(max_length=40)


m = UserModel()

# 调用描述器中的set方法，其中self表示描述器对象本身，instance表示UserModel这个类创建的实例m
# value表示hh
m.name = 'hh'