def refresh(self, force=False):
        """Refreshes the `access_token` and sets the praw instance `reddit_client`
        with a valid one.

        :param force: Boolean. Refresh will be done only when last refresh was
            done before `EXPIRY_DURATION`, which is 3500 seconds. However
            passing `force` will overrides this and refresh operation will be
            done everytime.
        """
        if self._is_token_expired() or force:
            tokens = self._get_refresh_access()
            self.access_token = tokens['access_token']
            self.refresh_token = tokens['refresh_token']
            self._set_access_credentials()