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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def fragment(args):
 """ %prog fragment fastafile enzyme Cut the fastafile using the specified enzyme, and grab upstream and downstream nucleotide sequence along with the cut site. In this case, the sequences extracted are: |- PstI ============|=========== (-------) Sometimes we need to limit the size of the restriction fragments, for example the GBS protocol does not allow fragments larger than 800bp. |-PstI |- PstI |- PstI ~~~====|=============|==========~~~~~~~===|============ (---) (---) In this case, the second fragment is longer than 800bp, therefore the two ends are NOT extracted, as in the first fragment. """

    p = OptionParser(fragment.__doc__)
    p.add_option("--flank", default=150, type="int",
            help="Extract flanking bases of the cut sites [default: %default]")
    p.add_option("--full", default=False, action="store_true",
            help="The full extraction mode [default: %default]")
    opts, args = p.parse_args(args)

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

    fastafile, enzyme = args
    flank = opts.flank
    assert flank > 0
    extract = extract_full if opts.full else extract_ends
    tag = "full" if opts.full else "ends"

    assert enzyme in set(str(x) for x in AllEnzymes)
    fragfastafile = fastafile.split(".")[0] + \
        ".{0}.flank{1}.{2}.fasta".format(enzyme, flank, tag)
    enzyme = [x for x in AllEnzymes if str(x) == enzyme][0]

    f = Fasta(fastafile, lazy=True)
    fw = open(fragfastafile, "w")
    for name, rec in f.iteritems_ordered():
        a = Analysis([enzyme], rec.seq)
        sites = a.full()[enzyme]
        extract(rec, sites, flank, fw)

    logging.debug("Fragments written to `{0}`.".format(fragfastafile))