First, we define a couple of utility functions that we will reuse to validate the fields that are mandatory or must follow a specific format:

def required(value, message):
    if not value:
        raise ValueError(message)
    return value

def matches(value, regex, message):
    if value and not regex.match(value):
        raise ValueError(message)
    return value

Then, we define our Contact class and its __init__ method. We set here all the parameters to the corresponding fields. We also store the compiled regular expressions as class attributes since we will use them for every instance to perform the field validations:

import re

class Contact(object):
    email_regex = re.compile(r"[^@]+@[^@]+.[^@]+")
    phone_regex = re.compile(r"([0-9]{3})s[0-9]{7}")

    def __init__(self, last_name, first_name, email, phone):
        self.last_name = last_name
        self.first_name = first_name
        self.email = email
        self.phone = phone

However, this definition is not enough to enforce the validations for each field. To do so, we use the @property decorator, which allow us to wrap access to an internal attribute:

    @property
    def last_name(self):
        return self._last_name

    @last_name.setter
    def last_name(self, value):
        self._last_name = required(value, "Last name is required")

The same technique is applied for first_name since it is also mandatory. The email and phone attributes follow a similar approach, using the matches function with the corresponding regular expression:

    @property
    def email(self):
        return self._email

    @email.setter
    def email(self, value):
        self._email = matches(value, self.email_regex,
                              "Invalid email format")

This script should be saved as chapter5_01.py, since we will import it later in future recipes with this name.