Python argparse help-like option

Question

I am writing a python script that takes two arguments, and some options:

scriptname [-h] [-l] [-q|-d] arg1 arg2

The -q (quiet) and -d (debug) options change the verbosity level, and the -h option is the help option automatically created by argparse.

I would like the -l (list) option to behave similarly to the -h option in that it will not require that the (otherwise mandatory) arguments are present and list some useful information (different from the -h option). In practice, this means that the script could be called in the following three ways:

scriptmane [-q|-d] arg1 arg2
scriptname -l
scriptname -h

Two possible ways forward would be to:

1. Make the arguments optional (with nargs='?') and add code to verify that there are two arguments in all cases where there -l og -h options are not given.
2. Write a custom action class (not sure about the details).

But I hope there is a more straightforward way to inherit the "this option is all you need" behaviour from the help option.

---

Solution (based on samwyse's answer):

Based on the _HelpAction() in argparse.py:

class _ListAction(argparse.Action):
    def __init__(self,
                 option_strings,
                 dest=argparse.SUPPRESS,
                 default=argparse.SUPPRESS,
                 help=None):
        super(_ListAction, self).__init__(
            option_strings=option_strings,
            dest=dest,
            default=default,
            nargs=0,
            help=help)

    def __call__(self, parser, namespace, values, option_string=None):
        print_list()
        parser.exit()

and then, during parser setup:

    parser.add_argument('-l', '--list', action=_ListAction, 
                        help="List all available cases")

Answer 1

If the option "list" is intended to have different behavior from "help" then you need to write a custom action. The good news is that it is very simple to do this. The main page for argparse gives you hints, you only have to realize that the action is called as soon as the option is seen in the list of arguments. In the new action's call (that should have two underscores at each end but markdown uses those for emphasis) method, do whatever you need to do for your option and then call parser.exit() to short circuit the processing of any more arguments.

Look at the source for _HelpAction and _VersionAction here: https://github.com/ThomasWaldmann/argparse/blob/master/argparse.py (and probably just subclass one of them to skip writing the init code).

Answer 2

You can easily do this with argparse. The following will support both -l and --list as additional help options:

parser.add_argument("-l", "--list", action="help")

If you don't want to support the long form (--list), simply omit it from the argument list.