python argparse - either both optional arguments or else neither one

Question

I have a program that uses a default name and password. I'm using argparse to allow the user to specify command line options, and I would like to enable the user to provide the program with a different name and password to use. So I have the following:

parser.add_argument(
    '-n',
    '--name',
    help='the login name that you wish the program to use'
    )

parser.add_argument(
    '-p',
    '--password',
    help='the password to log in with.'
    )

But it doesn't make any sense to specify only the name or only the password, but it would make sense to specify neither one. I noticed that argparse does have the ability to specify that two arguments are mutually exclusive. But what I have are two arguments that must appear together. How do I get this behavior? (I found "argument groups" mentioned in the docs, but they don't appear to solve my problem http://docs.python.org/2/library/argparse.html#argument-groups)

Answer 1

I believe that the best way to handle this is to post-process the returned namespace. The reason that argparse doesn't support this is because it parses arguments 1 at a time. It's easy for argparse to check to see if something was already parsed (which is why mutually-exclusive arguments work), but it isn't easy to see if something will be parsed in the future.

A simple:

parser.add_argument('-n','--name',...,default=None)
parser.add_argument('-p','--password',...,default=None)
ns = parser.parse_args()

if len([x for x in (ns.name,ns.password) if x is not None]) == 1:
   parser.error('--name and --password must be given together')

name = ns.name if ns.name is not None else "default_name"
password = ns.password if ns.password is not None else "default_password"

seems like it would suffice.

Answer 2

I know this is more than two years late, but I found a nice and concise way to do it:

if bool(ns.username) ^ bool(ns.password):
    parser.error('--username and --password must be given together')

^ is the XOR operator in Python. To require both arguments given at the command line is essentially an XOR test.

Answer 3

This is probably how I'd do it. Since you have existing defaults with the option to change, define the defaults, but don't use them as your argument defaults:

default_name = "x"
default_pass = "y"
parser.add_argument(
    '-n',
    '--name',
    default=None,
    help='the login name that you wish the program to use'
    )

parser.add_argument(
    '-p',
    '--password',
    default=None,
    help='the password to log in with.'
    )
args = parser.parse_args()
if all(i is not None for i in [args.name, args.password]):
    name = args.name
    passwd = args.password
elif any(i is not None for i in [args.name, args.password]):
    parser.error("Both Name and Password are Required!")
else:
    name = default_name
    passwd = default_pass