argparse metavar for nargs='+' to get numbered arguments in help info?

Question

In the argparse document, there is an example using nargs='+':

parser.add_argument('integers', metavar='N', type=int, nargs='+')

The corresponding help information is

usage: prog.py [-h] [--sum] N [N ...]

Is it possible to set the metavar such that the help info looks like this?

usage: prog.py [-h] [--sum] N1 [N2 ...]

Answer 1

metavar can be tuple, as in

In [26]: parser = argparse.ArgumentParser()
In [27]: a=parser.add_argument('--integers', metavar=('N1','N2'), type=int, nargs='+');
In [28]: parser.print_help()
usage: ipython3 [-h] [--integers N1 [N2 ...]]

optional arguments:
  -h, --help            show this help message and exit
  --integers N1 [N2 ...]

But that only works for optionals (flagged arguments), not positionals. I'm not exactly sure why, though I may have dug into this issue before.

https://docs.python.org/3/library/argparse.html#metavar

---

With the positional, the error stack includes:

/usr/lib/python3.5/argparse.py in _format_action_invocation(self, action)
    542         if not action.option_strings:
    543             default = self._get_default_metavar_for_positional(action)
--> 544             metavar, = self._metavar_formatter(action, default)(1)
    545             return metavar
    546 

A positional doesn't have option_strings (or its empty), so it's using the metavar, = ... unpacking idiom to ask for just one string. Hence the error: ValueError: too many values to unpack (expected 1).

So expecting only one metavar variable is quite intentional, but I'm not sure about the rational. Maybe it's because it would be harder to tell where one positional ends and the next starts in the usage:

In [34]: parser = argparse.ArgumentParser()
In [35]: parser.add_argument('foo', type=int, nargs=2);
In [36]: parser.add_argument('bar', type=int, nargs='+');
In [38]: parser.print_usage()
usage: ipython3 [-h] foo foo bar [bar ...]

I looked at this a couple of years ago, https://bugs.python.org/issue14074. Looks like there can be problems with the usage, help-lines and error message.

---

Actually the problem isn't in the usage, it's in formatting the help lines:

In [39]: parser = argparse.ArgumentParser()
In [40]: a=parser.add_argument('integers', metavar=('N1','N2'), type=int, nargs='+')
In [41]: parser.print_usage()
usage: ipython3 [-h] N1 [N2 ...]

The normal help line for a positional just shows one symbol, not the N [N ...] of the usage:

In [43]: parser.print_help()
usage: ipython3 [-h] N [N ...]

positional arguments:
  N

Answer 2

I would make a custom HelpFormatter and use that. Below is a quick hack.

from argparse import HelpFormatter, ZERO_OR_MORE, ONE_OR_MORE


class CustomHelpFormatter(HelpFormatter):
    def _format_args(self, action, default_metavar):
        get_metavar = self._metavar_formatter(action, default_metavar)
        if action.nargs == ZERO_OR_MORE:
            return '[%s1 [%s2 ...]]' % get_metavar(2)
        elif action.nargs == ONE_OR_MORE:
            return '%s1 [%s2 ...]' % get_metavar(2)
        else:
            return super()._format_args(action, default_metavar)


>>> parser = argparse.ArgumentParser(formatter_class=CustomHelpFormatter)
>>> parser.add_argument('integers', metavar='N', type=int, nargs='+')
>>> parser.parse_args(['--help'])