def set_variant(self, identity, experiment_name, variant_name):
        """
        Set the variant for a specific user.

        :param identity a unique user identifier
        :param experiment_name the string name of the experiment
        :param variant_name the string name of the variant
        """
        try:
            experiment = model.Experiment.get_by(name=experiment_name)
            variant = model.Variant.get_by(name=variant_name)
            if experiment and variant and model.Participant.query.filter(and_(
                model.Participant.identity == identity,
                model.Participant.experiment_id == experiment.id,
                model.Participant.variant_id == variant.id
            )).count() == 0:
                model.Participant(
                    identity=identity,
                    experiment=experiment,
                    variant=variant
                )
                self.Session.commit()
        finally:
            self.Session.close()
Set the variant for a specific user.

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