code: def exon_by_id(self, exon_id):
          
          if exon_id not in self._exons:
              field_names = [
                  "seqname",
                  "start",
                  "end",
                  "strand",
                  "gene_name",
                  "gene_id",
              ]
  
              contig, start, end, strand, gene_name, gene_id = self.db.query_one(
                  select_column_names=field_names,
                  filter_column="exon_id",
                  filter_value=exon_id,
                  feature="exon",
                  distinct=True)
  
              self._exons[exon_id] = Exon(
                  exon_id=exon_id,
                  contig=contig,
                  start=start,
                  end=end,
                  strand=strand,
                  gene_name=gene_name,
                  gene_id=gene_id)
  
          return self._exons[exon_id]
docs: Construct an Exon object from its ID by looking up the exon"s
          properties in the given Database.