def cache_master_key(config):
    """Create a string determined by any cache-invalidating config elements."""
    return (
        "str{use_string}_"
        "vocab{vocab_size}_"
        "pg{use_page}_"
        "geom{use_geom}_"
        "amt{use_amount}_"
        "pad{pad_windows}_"
        "len{window_len}"
    ).format(**config)