code: def _init_lazy_fields(self):
          
          self.gtf_path = None
          self._protein_sequences = None
          self._transcript_sequences = None
          self._db = None
          self.protein_fasta_paths = None
          self.transcript_fasta_paths = None
  
          # only memoizing the Gene, Transcript, and Exon objects
          self._genes = {}
          self._transcripts = {}
          self._exons = {}
docs: Member data that gets loaded or constructed on demand