Retrieve the variant for a specific user and experiment (if it exists).

        :param identity a unique user identifier
        :param experiment_name the string name of the experiment

        Returns a ``String`` or `None`

def get_variant(self, identity, experiment_name):
        """
        Retrieve the variant for a specific user and experiment (if it exists).

        :param identity a unique user identifier
        :param experiment_name the string name of the experiment

        Returns a ``String`` or `None`
        """
        try:
            match = model.Participant.query.join(
                model.Experiment
            ).filter(and_(
                model.Participant.identity == identity,
                model.Experiment.name == experiment_name
            )).first()
            return match.variant.name if match else None
        finally:
            self.Session.close()