code: def sort(args):
      valid_sort_methods = ("unix", "topo")
      p = OptionParser(sort.__doc__)
      p.add_option("--method", default="unix", choices=valid_sort_methods,
                   help="Specify sort method [default: %default]")
      p.add_option("-i", dest="inplace", default=False, action="store_true",
                   help="If doing a unix sort, perform sort inplace [default: %default]")
      p.set_tmpdir()
      p.set_outfile()
      p.set_home("gt")
      opts, args = p.parse_args(args)
      if len(args) != 1:
          sys.exit(not p.print_help())
      gffile, = args
      sortedgff = opts.outfile
      if opts.inplace:
          if opts.method == "topo" or (opts.method == "unix" and gffile in ("-", "stdin")):
              logging.error("Cannot perform inplace sort when method is `topo`" + \
                  " or method is `unix` and input is `stdin` stream")
              sys.exit()
      if opts.method == "unix":
          cmd = "sort"
          cmd += " -k1,1 -k4,4n {0}".format(gffile)
          if opts.tmpdir:
              cmd += " -T {0}".format(opts.tmpdir)
          if opts.inplace:
              cmd += " -o {0}".gffile
              sortedgff = None
          sh(cmd, outfile=sortedgff)
      elif opts.method == "topo":
          GT_HOME = opts.gt_home
          if not op.isdir(GT_HOME):
              logging.error("GT_HOME={0} directory does not exist".format(GT_HOME))
              sys.exit()
          cmd = "{0}".format(op.join(GT_HOME, "bin", "gt"))
          cmd += " gff3 -sort -tidy -retainids -addids no {0}".format(gffile)
          sh(cmd, outfile=sortedgff)