def _parse_known_args(p, *args, **kwargs):
    """_parse_known_args wraps argparse.ArgumentParser._parse_known_args
        to trigger defered functions once after parsing."""
    res = super()._parse_known_args(*args, **kwargs)
    fns = p.callbacks or []
    p.callbacks = None
    for fn in fns:
        fn()
    return res
