Optional stdin in Python with argparse

The standard library documentation for argparse suggests this solution to allow optional input/output files:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('infile', nargs="?", type=argparse.FileType('r'),
...                     default=sys.stdin)
>>> parser.add_argument('outfile', nargs="?", type=argparse.FileType('w'),
...                     default=sys.stdout)
>>> parser.parse_args(['input.txt', 'output.txt'])
Namespace(infile=<_io.TextIOWrapper name="input.txt" encoding='UTF-8'>,
          outfile=<_io.TextIOWrapper name="output.txt" encoding='UTF-8'>)
>>> parser.parse_args([])
Namespace(infile=<_io.TextIOWrapper name="<stdin>" encoding='UTF-8'>,
          outfile=<_io.TextIOWrapper name="<stdout>" encoding='UTF-8'>)