Three possible ways to implement the descriptor protocol in Python.

descriptors_decorator.py

import re


class User(object):

    def __init__(self):
        self._phone_number = ''

    @property
    def phone_number(self):
        return self._phone_number

    @phone_number.setter
    def phone_number(self, value):
        if re.match(r'^(\d{3})-(\d{3})-(\d{4})', value):
            print('Adding phone number for user: {}'.format(value))
            self._phone_number = value
        else:
            raise ValueError('Phone Number format does not match.')

    @phone_number.deleter
    def phone_number(self):
        print("Deleting: {}".format(self._phone_number))
        del self._phone_number


if __name__ == '__main__':
    py_user = User()
    py_user.phone_number
    py_user.phone_number = '800-123-4567'
    del py_user.phone_number

descriptors_override_methods.py

import re


class PyDescriptor(object):

    def __init__(self):
        self._name = None

    def __get__(self, instance, owner):
        print("Getting: {}".format(self._name))
        return self._name

    def __set__(self, instance, value):
        print("Setting: {}".format(self._name))
        print("To value: {}".format(value))
        self._name = value

    def __delete__(self, instance):
        print("Deleting: {}".format(self._name))
        del self._name 


class User(object):
    name = PyDescriptor()


if __name__ == '__main__':
    py_user = User()
    py_user.name
    py_user.name = 'Harry'
    del py_user.name

descriptors_property.py

import re


class User(object):

    def __init__(self):
        self._phone_number = ''

    def fget(self):
        return self._phone_number

    def fset(self, value):
        if re.match(r'^(\d{3})-(\d{3})-(\d{4})', value):
            print('Adding phone number for user: {}'.format(value))
            self._phone_number = value
        else:
            raise ValueError('Phone Number format does not match.')

    def fdel(self):
        print("Deleting: {}".format(self._phone_number))
        del self._phone_number

    phone_number = property(fget, fset, fdel, 'I am the phone number property.')


if __name__ == '__main__':
    user = User()
    user.phone_number = '123'