def time_cache(time_add_setting):
    """ This decorator works as follows: Call it with a setting and after that
    use the function with a callable that returns the key.
    But: This function is only called if the key is not available. After a
    certain amount of time (`time_add_setting`) the cache is invalid.
    """
    def _temp(key_func):
        dct = {}
        _time_caches.append(dct)

        def wrapper(optional_callable, *args, **kwargs):
            key = key_func(*args, **kwargs)
            value = None
            if key in dct:
                expiry, value = dct[key]
                if expiry > time.time():
                    return value
            value = optional_callable()
            time_add = getattr(settings, time_add_setting)
            if key is not None:
                dct[key] = time.time() + time_add, value
            return value
        return wrapper
    return _temp