Update 2: From the ArgParse documentation,
One of the more common uses of nargs='?' is to allow optional input and 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=<open file 'input.txt', mode 'r' at 0x...>,
outfile=<open file 'output.txt', mode 'w' at 0x...>)
>>> parser.parse_args([])
Namespace(infile=<open file '<stdin>', mode 'r' at 0x...>,
outfile=<open file '<stdout>', mode 'w' at 0x...>)
Original answer:
This is straightforward: just add a positional argument with a default value and a nargs='*'. The default value will be used if there are zero arguments, otherwise the arguments on the command line will be used:
>>> p = argparse.ArgumentParser()
>>> p.add_argument('filename', nargs='*', default=['-'])
>>> p.parse_args([])
Namespace(filename=['-'])
>>> p.parse_args(['abc'])
Namespace(filename=['abc'])
Typically, - is used to refer to standard input / standard output.
Then you do something like this:
def get_inputs(ns):
"""Iterate over input files."""
for path in ns.filename:
if path == '-':
yield sys.stdin
else:
yield open(path, 'r')
Update: I assumed you wanted multiple filenames, since cat takes multiple filenames. You can use nargs='?' if you want either zero or one filename.
