Executes the callable registered at the specified key and returns
            its value. Subsequent queries are cached internally.

            `key`
                String key for a previously stored callable.

def get(self, key):
        """ Executes the callable registered at the specified key and returns
            its value. Subsequent queries are cached internally.

            `key`
                String key for a previously stored callable.
            """

        if not key in self._actions:
            return None
        if not key in self._cache:
            self._cache[key] = self._actions[key]()

        return self._cache[key]