How To Blast Against A Local Database

I want to BLAST an mRNA reference sequence against a local database in my directory (a collection of RNA reference sequences) using a BioPython program. I know how to perform BLAST against online database such as NCBI database using the following code:

from Bio.Blast import NCBIWWW

sequence = """GGAGGATATATTCAAC"""

blast_handle = NCBIWWW.qblast('blastn', 'nr', sequence)

    blast_handle.seek(0)
    blast_file = open('blast-output.xml', 'w')
    blast_file.write(blast_handle.read())
    blast_file.close()

But my question is how to do BLAST against a local database in my directory?