def lookup_value(self, api_name, key):
        """Add the value of an API call to the cache.

        Args:
            api_name: a string name of the API. Keys and values are segmented by api_name.
            key: a string key for the specific call.
        """
        if api_name in self._cache:
            return self._cache[api_name].get(key, None)
        return None