code: def do_execute(self):
          
          data = self.input.payload
          if self.storagehandler is None:
              return "No storage handler available!"
          sname = str(self.resolve_option("storage_name"))
          if sname not in self.storagehandler.storage:
              return "No storage item called '" + sname + "' present!"
          cls = self.storagehandler.storage[sname]
          if isinstance(cls, Classifier):
              evl = Evaluation(data)
              evl.discard_predictions = bool(self.resolve_option("discard_predictions"))
              evl.test_model(
                  cls,
                  data,
                  self.resolve_option("output"))
          elif isinstance(cls, Clusterer):
              evl = ClusterEvaluation()
              evl.set_model(cls)
              evl.test_model(data)
          else:
              return "Unhandled class: " + classes.get_classname(cls)
          self._output.append(Token(evl))
          return None
docs: The actual execution of the actor.
  
          :return: None if successful, otherwise error message
          :rtype: str