import argparse


def argparser_from_spec(spec, **argparser_kwargs):
    ap = argparse.ArgumentParser(**argparser_kwargs)

    for line in spec.splitlines():
        line = line.strip()
        if not line:
            continue

        kwargs = {}
        optdefs, _sep, help = line.partition(":")

        help = help.strip()
        if help:
            kwargs["help"] = help

        items = optdefs.split()
        if len(items) == 1 and not items[0].startswith("-"):
            ap.add_argument(items[0], **kwargs)
            continue

        argnames = []
        metavars = []
        for i, optdef in enumerate(optdefs.split()):
            if optdef.startswith("-"):
                argnames.append(optdef)
            else:
                metavars.append(optdef)

        if len(metavars) == 0:
            kwargs["action"] = "store_true"
        elif len(metavars) == 1:
            kwargs["metavar"] = metavars[0]
        else:
            kwargs["metavar"] = tuple(metavars)
            kwargs["nargs"] = len(metavars)

        ap.add_argument(*argnames, **kwargs)

    return ap


spec = """
    positional: this is a positional argument
    -f --flag: this is a boolean flag argument
    -c --count N: this is an optional argument
    -s --size X Y: this is an optional argument with multiple parameters
"""
argparser = argparser_from_spec(spec, allow_abbrev=False)
print(argparser.parse_args("spam -f --count 42 -s 4 2".split()))