argparse argument dependency

Question

If I call the script below with these options:

--user u1 --password p1 --foo f1   --user u2   --user u3 --password p3

Then it will print:

Namespace(foo=['bar', 'f1'], password=['p1', 'p3'], user=['u1', 'u2', 'u3'])

Question: Is there any way for me to set up a dependency between user and password, so it throws an error, because password for user u2 is not specified?

Less relevant question: How do I specify a default foo value for all users? With the given input I would like foo to equal ['f1','bar','bar'].

A solution for my main question would be to check that the lists user and password have the same length, but it's not quite what I'm looking for.

Here is the script:

import argparse
parser = argparse.ArgumentParser()
group = parser.add_argument_group('authentication')
group.add_argument('--user', action='append', required=True)
group.add_argument('--password', action='append', required=True)
group.add_argument('--foo', action='append', default=['bar'])
print(parser.parse_args())

Answer 1

In your case, since the options must always be specified together, or none of them, you could join them into a unique --user-and-password option with two arguments using nargs=2. This would simplify a lot the handling of the values.

In fact you want to be able to provide multiple pairs, but required=True is satisfied when the first option is found, so it's pretty much useless for checking what you want in your setting.

The other way to do this is to use a custom action. For example:

import argparse


class UserAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        if len(namespace.passwords) < len(namespace.users):
            parser.error('Missing password')
        else:
            namespace.users.append(values)


class PasswordAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        if len(namespace.users) <= len(namespace.passwords):
            parser.error('Missing user')
        else:
            namespace.passwords.append(values)


parser = argparse.ArgumentParser()
parser.add_argument('--password', dest='passwords', default=[], action=PasswordAction, required=True)
parser.add_argument('--user', dest='users', default=[], action=UserAction, required=True)

print(parser.parse_args())

Used as:

$python3 ./test_argparse.py --user 1 --password 2 --password 2 --user 3 --password 3
usage: test_argparse.py [-h] --password PASSWORDS --user USERS
test_argparse.py: error: Missing user

And:

$python3 ./test_argparse.py --user 1 --password 2 --user 2 --user 3 --password 3
usage: test_argparse.py [-h] --password PASSWORDS --user USERS
test_argparse.py: error: Missing password

(Note that this solution requires --user to come before --password, otherwise the lengths of the lists don't provide enough information to understand when an option is missing.)

The last solution would be to simply use action='append' and test at the end the lists of values. However this would allow things like --user A --user B --password A --password B which may or may not be something you want to allow.

Answer 2

Define a custom user type which holds both username and password.

def user(s):
    try:
        username, password = s.split()
    except:
        raise argparse.ArgumentTypeError('user must be (username, password)')

group.add_argument('--user', type=user, action='append')