code: def sort(args):
      p = OptionParser(sort.__doc__)
      p.add_option("--sizes", default=False, action="store_true",
                   help="Sort by decreasing size [default: %default]")
      opts, args = p.parse_args(args)
      if len(args) != 1:
          sys.exit(p.print_help())
      fastafile, = args
      sortedfastafile = fastafile.rsplit(".", 1)[0] + ".sorted.fasta"
      f = Fasta(fastafile, index=False)
      fw = must_open(sortedfastafile, "w")
      if opts.sizes:
          sortlist = sorted(f.itersizes(), key=lambda x: (-x[1], x[0]))
          logging.debug("Sort by size: max: {0}, min: {1}".\
                          format(sortlist[0], sortlist[-1]))
          sortlist = [x for x, s in sortlist]
      else:
          sortlist = sorted(f.iterkeys())
      for key in sortlist:
          rec = f[key]
          SeqIO.write([rec], fw, "fasta")
      logging.debug("Sorted file written to `{0}`.".format(sortedfastafile))
      fw.close()
      return sortedfastafile