docs: 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()