def _add_dry_run(arg_parser, help_text):
    """
    Adds optional --dry-run parameter to a parser. Stored as 'dry_run'.
    :param arg_parser: ArgumentParser parser to add this argument to.
    :param help_text: str label displayed in usage
    """
    arg_parser.add_argument('--dry-run', help=help_text, action=
        'store_true', dest='dry_run')
