def prepare(args):
    """
    %prog prepare pairsfile cdsfile [pepfile] -o paired.cds.fasta

    Pick sequences from cdsfile to form pairs, ready to be calculated. The
    pairsfile can be generated from formats.blast.cscore(). The first two
    columns contain the pair.
    """
    from jcvi.formats.fasta import Fasta

    p = OptionParser(prepare.__doc__)
    p.set_outfile()

    opts, args = p.parse_args(args)
    outfile = opts.outfile

    if len(args) == 2:
        pairsfile, cdsfile = args
        pepfile = None
    elif len(args) == 3:
        pairsfile, cdsfile, pepfile = args
    else:
        sys.exit(not p.print_help())

    f = Fasta(cdsfile)
    fp = open(pairsfile)
    fw = must_open(outfile, "w")
    if pepfile:
        assert outfile != "stdout", "Please specify outfile name."
        f2 = Fasta(pepfile)
        fw2 = must_open(outfile + ".pep", "w")
    for row in fp:
        if row[0] == '#':
            continue
        a, b = row.split()[:2]
        if a == b:
            logging.debug("Self pairs found: {0} - {1}. Ignored".format(a, b))
            continue

        if a not in f:
            a = find_first_isoform(a, f)
            assert a, a
        if b not in f:
            b = find_first_isoform(b, f)
            assert b, b

        acds = f[a]
        bcds = f[b]
        SeqIO.write((acds, bcds), fw, "fasta")
        if pepfile:
            apep = f2[a]
            bpep = f2[b]
            SeqIO.write((apep, bpep), fw2, "fasta")
    fw.close()
    if pepfile:
        fw2.close()