python argparse choices with a default choice

I'm trying to use argparse in a Python 3 application where there's an explicit list of choices, but a default if none are specified.

The code I have is:

parser.add_argument('--list', default='all', choices=['servers', 'storage', 'all'], help='list servers, storage, or both (default: %(default)s)') 
args = parser.parse_args()
print(vars(args))

However, when I run this I get the following with an option:

$ python3 ./myapp.py --list all
{'list': 'all'}

Or without an option:

$ python3 ./myapp.py --list
usage: myapp.py [-h] [--list {servers,storage,all}]
myapp.py: error: argument --list: expected one argument

Am I missing something here? Or can I not have a default with choices specified?

Solution 1:

Pass the nargs and const arguments to add_argument:

parser.add_argument('--list',
                    default='all',
                    const='all',
                    nargs='?',
                    choices=['servers', 'storage', 'all'],
                    help='list servers, storage, or both (default: %(default)s)')

If you want to know if --list was passed without an argument, remove the const argument, and check if args.list is None.

Solution 2:

Thanks @ShadowRanger. Subcommands is exactly what I need, combined with nargs and const. The following works:

parser = argparse.ArgumentParser()
subparser = parser.add_subparsers()
parser_list = subparser.add_parser('list')
parser_list.add_argument('list_type', default='all', const='all', nargs='?', choices=['all', 'servers', 'storage'])

parser_create = subparser.add_parser('create')
parser_create.add_argument('create_type', default='server', const='server', nargs='?', choices=['server', 'storage'])

args = parser.parse_args()
pprint(vars(args))

$ python3 ./myapp.py -h
usage: dotool.py [-h] {list,create} ...

Digital Ocean tool

positional arguments:
  {list,create}

optional arguments:
  -h, --help     show this help message and exit

list option alone:

$ python3 ./myapp.py list
{'list_type': 'all'}

List option with a parameter:

$ python3 ./myapp.py list servers
{'list_type': 'servers'}