class user:
    self.username = ''
    self.password = ''
    self.email = ''
    def register(login, password, email):
        ...some actions
        user_object = user()
        user_object.username = login
        user_object.password = password
        user_object.email = email
        return user_object

    def set_username (self, username):
        self.username = username

u = user().register('login', 'password', 'mail@example.com')
print u.username // выведет login
u.set_username ('login2')
print u.username // выведет login2