This is a feature request for options that would work like the "find" program's "-exec" option, which takes the arguments following it unconditionally until it encounters a ";" or a "+".

There are multiple ways to achieve this, but I think a good general solution would be to let "nargs" take a function. This function would receive a list of all arguments following the current one and return the number of arguments that the current one should take. An example usage would be:

    def command_nargs(args_after):
        for i, arg in enumerate(args_after):
            if arg in [';', '+']:
                return i + 1
        raise argparse.ArgumentTypeError(
            'Expected a command terminated with `;` or `+`.')

    parser = argparse.ArgumentParser()
    parser.add_argument('--command', nargs=command_nargs)
    parser.add_argument('--option')

    parser.parse_args(['--command', 'echo', 'hello', ';', '--option', 'value'])
    # => Namespace(command=['echo', 'hello', ';'], option='value')

    parser.parse_args(['--command', 'program', '--option', '+'])
    # => Namespace(command=['program', '--option', '+'], option=None)

The crucial difference between this and the current behavior of "nargs" is that the requested number of arguments would be taken unconditionally, even if the arguments looked like options:

    parser = argparse.ArgumentParser()
    parser.add_argument('--nargs-function', nargs=lambda _: 1)
    parser.add_argument('--nargs-number', nargs=1)
    parser.add_argument('--option')

    parser.parse_args(['--nargs-function', '--option'])
    # => Namespace(nargs_function=['--option'], option=None)

    parser.parse_args(['--nargs-number', '--option'])
    # => error: argument --nargs-number: expected 1 argument

Alternatively, if the difference between the behavior of "nargs=number" and "nargs=function" is judged to be too large, an optional argument "greedy" could be added to "ArgumentParser" and "add_argument".

With "greedy=False" (the default), argparse would behave like it currently does. With "greedy=True", options would be processed left to right and would be assigned values unconditionally, even if these values looked like other options. An option with "nargs='*', greedy=True" or "nargs='+', greedy=True" would take all arguments to its right. An option with "nargs=function, greedy=False" would stop taking arguments when it encountered an option, while with "greedy=True" it would have the behavior proposed in the previous paragraphs.

An example usage would then be:

    parser = argparse.ArgumentParser()
    parser.add_argument('--nargs-function', nargs=lambda _: 1)
    parser.add_argument('--nargs-number', nargs=1)
    parser.add_argument('--option')

    parser.parse_args(['--nargs-function', '--option'])
    # => error: argument --nargs-function: expected 1 argument

    parser.parse_args(['--nargs-number', '--option'])
    # => error: argument --nargs-number: expected 1 argument

    ####################

    parser = argparse.ArgumentParser()
    parser.add_argument('--nargs-function', nargs=lambda _: 1, greedy=True)
    parser.add_argument('--nargs-number', nargs=1, greedy=True)
    parser.add_argument('--option')

    parser.parse_args(['--nargs-function', '--option'])
    # => Namespace(nargs_function=['--option'], option=None)

    parser.parse_args(['--nargs-number', '--option'])
    # => Namespace(nargs_number=['--option'], option=None)

and:

    parser = argparse.ArgumentParser()
    parser.add_argument('--command', nargs=command_nargs)
    parser.add_argument('--option')
    parser.add_argument('arg', nargs='?')

    parser.parse_args(['--command', 'program', '--option', 'value', '+'])
    # => Namespace(arg='+', command=['program'], option='value')

    ####################

    parser = argparse.ArgumentParser(greedy=True)
    parser.add_argument('--command', nargs=command_nargs)
    parser.add_argument('--option')
    parser.add_argument('arg', nargs='?')

    parser.parse_args(['--command', 'program', '--option', 'value', '+'])
    # => Namespace(arg=None, command=['program', '--option', 'value', '+'], option=None)

Both of the alternatives in this feature request have the upside of making the behaviors requested in #9334 and #14364 possible.