Get the follower list of a logged-in user.

        Parameters
        ----------
        auth_secret: str
            The authentication secret of the logged-in user.

        Returns
        -------
        bool
            True if the follower list is successfully obtained, False otherwise.
        result
            A dict containing the follower list with the key FOLLOWER_LIST_KEY
            if the follower list is successfully obtained, a dict containing
            the error string with the key ERROR_KEY otherwise.

        Note
        ----
        Possible error strings are listed as below:

        -  ERROR_NOT_LOGGED_IN

def get_followers(self, auth_secret):
        """Get the follower list of a logged-in user.

        Parameters
        ----------
        auth_secret: str
            The authentication secret of the logged-in user.

        Returns
        -------
        bool
            True if the follower list is successfully obtained, False otherwise.
        result
            A dict containing the follower list with the key FOLLOWER_LIST_KEY
            if the follower list is successfully obtained, a dict containing
            the error string with the key ERROR_KEY otherwise.

        Note
        ----
        Possible error strings are listed as below:

        -  ERROR_NOT_LOGGED_IN
        """
        result = {pytwis_constants.ERROR_KEY: None}

        # Check if the user is logged in.
        loggedin, userid = self._is_loggedin(auth_secret)
        if not loggedin:
            result[pytwis_constants.ERROR_KEY] = pytwis_constants.ERROR_NOT_LOGGED_IN
            return (False, result)

        # Get the list of followers' userids.
        follower_zset_key = pytwis_constants.FOLLOWER_KEY_FORMAT.format(userid)
        follower_userids = self._rc.zrange(follower_zset_key, 0, -1)

        if follower_userids is None or not follower_userids:
            result[pytwis_constants.FOLLOWER_LIST_KEY] = []
            return (True, result)

        # Get the list of followers' usernames from their userids.
        with self._rc.pipeline() as pipe:
            pipe.multi()

            for follower_userid in follower_userids:
                follower_userid_profile_key = \
                    pytwis_constants.USER_PROFILE_KEY_FORMAT.format(follower_userid)
                pipe.hget(follower_userid_profile_key, pytwis_constants.USERNAME_KEY)

            result[pytwis_constants.FOLLOWER_LIST_KEY] = pipe.execute()

        return (True, result)