code: def _codon_positions(self, feature):
          
          results = self._transcript_feature_positions(feature)
          if len(results) != 3:
              raise ValueError(
                  "Expected 3 positions for %s of %s but got %d" % (
                      feature,
                      self.id,
                      len(results)))
          return results
docs: Parameters
          ----------
          feature : str
              Possible values are "start_codon" or "stop_codon"
  
          Returns list of three chromosomal positions.