def next(self):
        command, sha = self.walker.read_proto_line(_GRAPH_WALKER_COMMANDS)
        if command in (None, 'done'):
            if not self._sent_ack:
                self.walker.send_nak()
            return None
        elif command == 'have':
            return sha
dataset/ETHPy150Open natestedman/Observatory/observatory/lib/dulwich/server.py/SingleAckGraphWalkerImpl.next/original