code: def _get_labels_left(self, validate=None):
          
  
          labels = []
  
          for compare_func in self.features:
  
              labels = labels + listify(compare_func.labels_left)
  
          # check requested labels (for better error messages)
          if not is_label_dataframe(labels, validate):
              error_msg = "label is not found in the dataframe"
              raise KeyError(error_msg)
  
          return unique(labels)
docs: Get all labels of the left dataframe.