code: def set_alternative(self, experiment_name, alternative):
          
          experiment = experiment_manager.get_experiment(experiment_name)
          if experiment:
              self._set_enrollment(experiment, alternative)
docs: Explicitly set the alternative the user is enrolled in for the specified experiment.
  
          This allows you to change a user between alternatives. The user and goal counts for the new
          alternative will be increment, but those for the old one will not be decremented. The user will
          be enrolled in the experiment even if the experiment would not normally accept this user.