argparse's help messages for variables that use "store_const" can probably be improved:
- propagate the default to all the options
- mark the default option as such explicitly
- group the keywords that have the same destination together (?)
- place the default option first (?)

For example, wouldn't the attached .py be better served with a help message that looks like:
...
optional arguments:
  -h, --help         show this help message and exit
  -w, --wrench       use a wrench (default)
  -H, --hammer       use a hammer (default: wrench)
  -s, --screwdriver  use a screwdriver (default: wrench)
  -T, --tnt          just blow the whole thing up (default: wrench)
  --foo FOO          foo bar (default: None)

?

Special casing the formatting of 'store_const' makes me nervous because formatting is already complicated and it doesn't know anything about action types.

But feel free to propose a patch!

In the meantime:

- propagate the default to all the options

You can, of course, do this manually, by specifying default in each of them. And if you use formatter_class=ArgumentDefaultsHelpFormatter, you'll get the defaults in the usage message.

- group the keywords that have the same destination together (?)
- place the default option first (?)

You can achieve both of these by just adding the options in the order that you want them displayed.