How to make a short and long version of a required argument using Python Argparse?

I want to specify a required argument called inputdir but I also would like to have a shorthand version of it called i. I don't see a concise solution to do this without making both optional arguments and then doing my own check. Is there a preferred practice for this that I'm not seeing or the only way is to make both optional and do my own error-handling?

Here is my code:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("inputdir", help="Specify the input directory")
parser.parse_args()

Solution

For flags (options starting with - or --) pass in options with the flags. You can specify multiple options:

parser.add_argument('-i', '--inputdir', help="Specify the input directory")

See the name or flags option documentation:

The add_argument() method must know whether an optional argument, like -f or --foo, or a positional argument, like a list of filenames, is expected. The first arguments passed to add_argument() must therefore be either a series of flags, or a simple argument name.

Demo:

>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('-i', '--inputdir', help="Specify the input directory")
_StoreAction(option_strings=['-i', '--inputdir'], dest='inputdir', nargs=None, const=None, default=None, type=None, choices=None, help='Specify the input directory', metavar=None)
>>> parser.print_help()
usage: [-h] [-i INPUTDIR]

optional arguments:
  -h, --help            show this help message and exit
  -i INPUTDIR, --inputdir INPUTDIR
                        Specify the input directory
>>> parser.parse_args(['-i', '/some/dir'])
Namespace(inputdir='/some/dir')
>>> parser.parse_args(['--inputdir', '/some/dir'])
Namespace(inputdir='/some/dir')

However, the first element for required arguments is just a placeholder. - and -- options are always optional (that's the command line convention), required arguments are never specified with such switches. Instead the command-line help will show where to put required arguments with a placeholder based on the first argument passed to add_argument(), which is to be passed in without dashes.

If you have to break with that convention and use an argument starting with - or -- that is required anyway, you'll have to do your own checking:

args = parser.parse_args()
if not args.inputdir:
    parser.error('Please specify an inputdir with the -i or --inputdir option')

Here the parser.error() method will print the help information together with your error message, then exit.