def cache_as_field(cache_name):
    """Cache a functions return value as the field 'cache_name'."""
    def cache_wrapper(func):
        @functools.wraps(func)
        def inner_wrapper(self, *args, **kwargs):
            value = getattr(self, cache_name, UndefToken)
            if value != UndefToken:
                return value

            ret = func(self, *args, **kwargs)
            setattr(self, cache_name, ret)
            return ret
        return inner_wrapper
    return cache_wrapper