def reference_id(self, index):
        """Return the reference ID (locally defined)."""
        # TODO Include ISBN and ISSN?
        doi = self.reference_doi(index)
        if doi:
            return doi
        else:
            pmid = self.reference_pmid(index)
            if pmid:
                return "PMID_" + pmid
            else:
                unpublished_id = self.reference_unpublished_id(index)
                if unpublished_id:
                    return "UNPUBLISHED_" + unpublished_id
        return ""
def reference_id ( self , index ) : doi = self . reference_doi ( index ) if doi : return doi else : pmid = self . reference_pmid ( index ) if pmid : return "PMID_" + pmid else : unpublished_id = self . reference_unpublished_id ( index ) if unpublished_id : return "UNPUBLISHED_" + unpublished_id return ""
Return the reference ID (locally defined).