code: def get_output_jsonpath_field(self, sub_output=None):
          
          if sub_output is not None:
              if self.output_fields is None or\
                  (isinstance(self.output_fields, dict) and not sub_output in self.output_fields.itervalues()) or\
                      (isinstance(self.output_fields, list) and not sub_output in self.output_fields):
                  raise ValueError(
                      "Cannot generate output jsonpath because this ExtractorProcessor will not output {}".format(sub_output))
              output_jsonpath_field = sub_output
          else:
              output_jsonpath_field = self.output_field
          return output_jsonpath_field
docs: attempts to create an output jsonpath from a particular ouput field