code: def __learn_labels(self, labels):
          
          if self.feature_length > 0:
              result = list(self.labels.classes_)
          else:
              result = []
  
          for label in labels:
              result.append(label)
          self.labels.fit(result)
docs: Learns new labels, this method is intended for internal use
  
          Args:
              labels (:obj:`list` of :obj:`str`): Labels to learn