def METHOD_NAME(self, user):
    """
    Add a vote to the choice from the given user.
    An exception is raised if the user has already voted.
    """
    if self.poll.user_has_voted(user):
        raise UserHasVoted(f"{user} has already voted on {self.poll}.")
    else:
        self.votes += 1
        self.save()
        self.poll.users_voted.add(user)