Argparse: ignore multiple positional arguments when optional argument is specified

Question

I'm trying to make argparse ignore the fact that two normally required positional arguments shouldn't be evaluated when an optional argument (-l) is specified.

Basically I'm trying to replicate the behavior of --help: when you specify the -h, all missing required arguments are ignored.

Example code:

parser = argparse.ArgumentParser(description="Foo bar baz")
parser.add_argument('arg1', help='arg1 is a positional argument that does this')
parser.add_argument('arg2', help='arg2 is a positional argument that does this')
parser.add_argument('-l', '--list', dest='list', help='this is an optional argument that prints stuff')

options, args = parser.parse_args()

if options.list:
   print "I list stuff"

And of course, if I run it now, I get :

error: too few arguments

I tried different things like nargs='?', but couldn't get anything working.

This question is quite similar but wasn't answered.

Answer 1

Unfortunately, argparse isn't quite flexible enough for this. The best you can do is to make arg1 and arg2 optional using nargs="?" and check yourself whether they are given if needed.

The internal help action is implemented by printing the help message and exiting the program as soon as -h or --help are encountered on the command line. You could write a similar action yourself, something like

class MyAction(argparse.Action):
    def __call__(self, parser, values, namespace, option_string):
        print "Whatever"
        parser.exit()

(Warning: untested code!)

There are definite downsides to the latter approac, though. The help message will unconditionally show arg1 and arg2 as compulsory arguments. And parsing the command line simply stops when encountering -l or --list, ignoring any further arguments. This behaviour is quite acceptable for --help, but is less than desirable for other options.

Answer 2

I ran into this issue and decided to use subcommands. Subcommands might be overkill, but if you find your program not using some of the positional arguments in many instances (as I did), then subcommands might be a good solution.

For your given example, I'd use something like the following:

parser = argparse.ArgumentParser(description="Foo bar baz")
subparsers = parser.add_subparsers(description='available subcommands')

parser_main = subparsers.add_parser('<main_command_name>')
parser_main.add_argument('arg1', help='arg1 is a positional argument that does this')
parser_main.add_argument('arg2', help='arg2 is a positional argument that does this')

parser_list = subparsers.add_parser('list', help='this is a subcommand that prints stuff')

options, args = parser.parse_args()

I left out some details that you might want to include (like set_defaults(func=list)), which are mentioned in the argparse documentation.