How to add optional or once arguments?

Question

How can I add an argument that is optional and must not be specified multiple times?

Valid:

$ ./my.py
$ ./my.py --arg MyArgValue

Invalid:

$ ./my.py --arg MyArgValue --arg ThisIsNotValid

If I add an argument like:

parser.add_argument('--arg', type=str)

The invalid example results in a string ThisIsNotValid. I would expect a parser error.

Answer 1

Create a custom action that raises an exception if the same argument is seen twice. When the parser catches the exception, it prints the usage and a nicely-formatted error message.

import argparse

class Highlander(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        if getattr(namespace, self.dest, None) is not None:
            raise argparse.ArgumentError(self, 'There can be only one.')
        setattr(namespace, self.dest, values)

parser = argparse.ArgumentParser()
parser.add_argument('-f', action=Highlander)
print (parser.parse_args('-f 1 -f 2'.split()))