<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def gb(args):
 """ %prog gb gffile fastafile Convert GFF3 to Genbank format. Recipe taken from: <http://www.biostars.org/p/2492/> """

    from Bio.Alphabet import generic_dna
    try:
        from BCBio import GFF
    except ImportError:
        print("You need to install dep first: $ easy_install bcbio-gff", file=sys.stderr)

    p = OptionParser(gb.__doc__)
    opts, args = p.parse_args(args)

    if len(args) != 2:
        sys.exit(not p.print_help())

    gff_file, fasta_file = args
    pf = op.splitext(gff_file)[0]
    out_file = pf + ".gb"
    fasta_input = SeqIO.to_dict(SeqIO.parse(fasta_file, "fasta", generic_dna))
    gff_iter = GFF.parse(gff_file, fasta_input)
    SeqIO.write(gff_iter, out_file, "genbank")