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.

Documention:

nargs with '?'

One argument will be consumed from the command line if possible, and produced as a single item. If no command-line argument is present, the value from default will be produced. Note that for optional arguments, there is an additional case - the option string is present but not followed by a command-line argument. In this case the value from const will be produced.

const

When add_argument() is called with option strings (like -f or --foo) and nargs='?'. This creates an optional argument that can be followed by zero or one command-line arguments. When parsing the command line, if the option string is encountered with no command-line argument following it, the value of const will be assumed instead. See the nargs description for examples.

import argparse


parser = argparse.ArgumentParser(description='PROJECT_NAME')
parser.add_argument(
    '--mode', '-m',
    help='Set mode',
    default=1,
    type=int,
    choices=[1,2,3,4],
)
args = parser.parse_args()

print(args.mode)

For a complete list of the options available visit the docs:

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

Update:

Added the suggestions from MaLiN2223's comment

I found a way to get the behavior you want, but with a different syntax than what you present. You have to specify each choice with a unique parameter name/value pair. If that's ok, then the following works:

parser = argparse.ArgumentParser(prog='game.py')
parser.add_argument('--p1', choices=['a', 'b', 'c'], action='append')
args = parser.parse_args(['--p1', 'a', '--p1', 'b'])
print(args)

Result:

Namespace(p1=['a', 'b'])

but this fails appropriately:

parser = argparse.ArgumentParser(prog='game.py')
parser.add_argument('--p1', choices=['a', 'b', 'c'], action='append')
args = parser.parse_args(['--p1', 'a', '--p1', 'b', '--p1', 'x'])
print(args)

Result:

usage: game.py [-h] [--p1 {a,b,c}]
game.py: error: argument --p1: invalid choice: 'x' (choose from 'a', 'b', 'c')

I can find nothing in the docs that suggests that ArgumentParser will take a list of valid values as a parameter value, which is what your version would require.

You can set type=str.lower.

See Case insensitive argparse choices

import argparse

parser = argparse.ArgumentParser(prog='game.py')
parser.add_argument('move', choices=['rock', 'paper', 'scissors'], type=str.lower)
parser.parse_args(['rOCk'])
# Namespace(move='rock')