The FileType factory creates objects that can be passed to the type argument of add_argument(). Arguments that have FileType objects as their type will open command-line args as files with the requested modes and buffer sizes:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--output', type=argparse.FileType('wb', 0))
>>> parser.parse_args(['--output', 'out'])
Namespace(output=<open file 'out', mode 'wb' at 0x...>)
FileType objects understand the pseudo-argument '-' and automatically convert this into sys.stdin for readable FileType objects and sys.stdout for writable FileType objects:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('infile', type=argparse.FileType('r'))
>>> parser.parse_args(['-'])
Namespace(infile=<open file '<stdin>', mode 'r' at 0x...>)