Display error when argument is specified multiple times

Question

When specifying the same argument multiple times, the default argparse behaviour is that the last specified value "wins", overwriting the previous value. I want argparse to show an error when the same argument is specified multiple times instead of silently overwriting the first specified value.

How can that be done?

My current code:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument(
    '-t', '--test',      # either of this switches
    type=str,            # parameter is string
    dest='test',         # store in 'test'.
    required=True        # required
)

Calling the script:

myscript.py -t hello -t world

leads to no errors, test has the value world. I want argparse to show an error in this case, as the default behaviour is error prone from my point of view.

georg · Accepted Answer · 2019-03-30T19:23:11.367

The simplest option is to use action="append" and check if the result has len > 1:

parser.add_argument(
    '-t', '--test',
    type=str,
    dest='test',
    action='append',
    required=True
)

args = parser.parse_args()

if len(args.test) > 1:
   ...error

But a nicer way would be to solve the problem within the parser, so that you get unified error handling. This can be done with a custom Action like this:

class Once(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        if hasattr(self, 'seen'):
            raise argparse.ArgumentError(self, 'only one please')
        setattr(self, 'seen', True)
        setattr(namespace, self.dest, values)


parser = argparse.ArgumentParser()
parser.add_argument(
    '-t', '--test',
    type=str,
    dest='test',
    action=Once,
    required=True
)

Thanks! Previously didn't find the duplicates, but after comparing duplicates and this one, I like this one more, the error by argpase.ArgumentError looks nicer to me. — stefan.at.kotlin, Mar 30 '19 at 15:40
Thanks, but see my update. `if getattr(namespace...` has problems with default arguments, an explicit flag appears to be cleaner. — georg, Mar 30 '19 at 19:24

Display error when argument is specified multiple times

1 Answers1