def absolute_path(string):
    '''"Convert" a string to a string that is an absolute existing path.'''
    if not os.path.isabs(string):
        msg = '{0!r} is not an absolute path'.format(string)
        raise argparse.ArgumentTypeError(msg)
    if not os.path.exists(os.path.dirname(string)):
        msg = 'path {0!r} does not exist'.format(string)
        raise argparse.ArgumentTypeError(msg)
    return string
"Convert" a string to a string that is an absolute existing path.