validate_descriptor.py

import re

class Name:
    def __get__(self, obj, objtype=None):
        return self.value

    def __set__(self, obj, value):
        if len(value) > 20:
            raise ValueError("Name cannot exceed 20 characters.")
        self.value = value

class Email:
    def __get__(self, obj, objtype=None):
        return self.value

    def __set__(self, obj, value):
        regex = "^[a-z0-9]+[\._]?[a-z0-9]+[@]\w+[.]\w{2,3}$"
        if not re.match(regex, value):
            raise ValueError("It's not an email address.")
        self.value = value

class Age:
    def __get__(self, obj, objtype=None):
        return self.value

    def __set__(self, obj, value):
        if value < 0:
            raise ValueError("Age cannot be negative.")
        self.value = value

class Citizen:

    name = Name()
    email = Email()
    age = Age()

    def __init__(self, id, name, email, age):
        self.id = id
        self.name = name
        self.email = email
        self.age = age

xiaoxu = Citizen("id1", "xiaoxu gao", "xiaoxugao@gmail.com", 27)
xiaoxu = Citizen("id1", "xiaoxu1234567890123456789", "highsmallxu@gmail.com", 27)
# ValueError: Name cannot exceed 20 characters.
xiaoxu = Citizen("id1", "xiaoxu gao", "highsmallxu@gmail.c", 27)
# ValueError: It's not an email address.
xiaoxu = Citizen("id1", "xiaoxu gao", "highsmallxu@gmail.com", -27)
# ValueError: Age cannot be negative.