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)

And I assume that post-processing the arguments is out the the question? — mgilson
– mgilson, Commented Jan 16, 2013 at 2:14
Nothing is out of the question. I just want argparse to do the work for me and tell the user that the options must appear together. — tadasajon
– tadasajon, Commented Jan 16, 2013 at 2:20

Ting Qian · Accepted Answer · 2015-10-07 17:11:41Z

27

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.

answered Oct 7, 2015 at 17:11

Ting Qian

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2 Comments

Kaustubh Karkare Over a year ago

Shouldn't this be XNOR?

Akif Over a year ago

@KaustubhKarkare XOR evaluated as True if only username or password is given. Above describes the failure case in such condition, so it should be XOR.

mgilson · Accepted Answer · 2013-01-16 02:35:04Z

25

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.

edited Jan 16, 2013 at 2:35

answered Jan 16, 2013 at 2:21

mgilson

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3 Comments

James Polley Over a year ago

This is correct, but 'name = ns.name or "default_name"' is simpler and in my opinion easier to read that 'ns.name if ns.name is not None else ...'. However, as mgilson points out elsewhere, if the user passes an empty string for either value, the empty string will evaluate false and the default will be used instead of the provided empty string.

wecsam Over a year ago

sum(1 for x in (ns.name, ns.password) if x is not None) also works in the place of len([x for x in (ns.name,ns.password) if x is not None]).

Danon Over a year ago

Wouldn't ns.name is None ^ ns.password is None be enough? :D

monkut · Accepted Answer · 2013-01-16 02:38:34Z

7

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

edited Jan 16, 2013 at 2:38

answered Jan 16, 2013 at 2:26

monkut

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5 Comments

mgilson Over a year ago

Careful with this one. What if the user sets up their password to an empty string?

mgilson Over a year ago

As a side note, you can always edit your deleted answer and un-delete it :)

monkut Over a year ago

Thanks, it is kind of annoying to have it hanging there.

mgilson Over a year ago

Well, more to the point, if you were to get 10 votes and the checkmark, you might get an enlightened badge :)

monkut Over a year ago

thanks, and updated with the check for None, instead of Falseness.

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