def __init__(self, *args, **kwargs):
        self.transcript_key = kwargs.pop('transcript_key', 'transcript_id')
        self.gene_key = kwargs.pop('gene_key', 'gene_id')
        self.subfeature = kwargs.pop('subfeature', 'exon')
        super(_GTFDBCreator, self).__init__(*args, **kwargs)
create_db() delegates to this class -- see that function for docs
dataset/ETHPy150Open daler/gffutils/gffutils/create.py/_GTFDBCreator.__init__