argparse - option with optional value

[Ben Finney]

Miki Tebeka <miki.tebeka at gmail.com> writes:

> I'd like to have an --edit option in my program. That if not specified
> will not open editor. If specified without value will open default
> editor ($EDITOR) and if specified with value, assume this value is the
> editor program to run.

So, two rather separate tasks: handle the command-line option, and start
the editor.

> The way I'm doing it currently is:
>     ...
>     no_edit = 'no-edit'
>     parser.add_argument('-e', '--edit', help='open editor on log', nargs='?',
>                         default=no_edit)

There is a built-in “no value specified” value in Python: the None
singleton. The ‘argparse’ library uses this for the argument default
already, so you don't need to fuss with your own special handling
<URL:http://docs.python.org/library/argparse.html#default>.

So the above becomes::

    parser.add_argument('-e', '--edit', help='open editor on log', nargs='?')

>     ...
>     if args.edit != no_edit:

and this test becomes the familiar test against None::

    if args.edit is not None:

>         editor = args.edit or environ.get('EDITOR', 'vim')

This will work fine, AFAICT.

-- 
 \            “But it is permissible to make a judgment after you have |
  `\    examined the evidence. In some circles it is even encouraged.” |
_o__)                    —Carl Sagan, _The Burden of Skepticism_, 1987 |
Ben Finney