存一个再取回来

用 roundtrip 先 PUT k v、再 GET k，第二次交回什么？

贯穿全条的内存版 TCP（判题机不联网；接口和真 socket 一样，真机上换成 socket.socket() 就是真的）：net = Net()；lis = listen(net, 端口) 开监听；cli = connect(net, 端口) 客户端连上（交回客户端这头，服务端那头进 accept 队列）；srv = lis.accept() 领一条连接；sendall 把字节追加进对面收缓冲，recv(n) 从自己收缓冲取最多 n 字节（拿到多少算多少，recv 交回 b"" 表示对面关了）。

贯穿的应用层协议（一行一命令、按换行切）：req(*parts) 用空格连、末尾加换行；decode(line) 拆成 (命令, 参数表)；handle(store, line)：PUT k v→OK、GET k→值或 NONE、DEL k→OK/NONE、其它→ERR。

客户端：send_req(cli,*parts) 发一条；read_reply(cli) 收一行回复去掉换行；roundtrip(net,port,store,*parts) 一次完整来回。

import collections


class Net:
    def __init__(self):
        self.listeners = {}


class Endpoint:
    def __init__(self, name):
        self.name = name
        self.rbuf = b""
        self.peer = None
        self.peer_closed = False
        self.closed = False

    def send(self, data):
        if self.closed or self.peer is None:
            raise BrokenPipeError("连接已关")
        self.peer.rbuf += bytes(data)
        return len(data)

    def sendall(self, data):
        self.send(data)

    def recv(self, bufsize):
        if self.rbuf:
            out, self.rbuf = self.rbuf[:bufsize], self.rbuf[bufsize:]
            return out
        if self.peer_closed:
            return b""
        raise BlockingIOError("暂时没有数据（真 socket 会在这里阻塞等）")

    def close(self):
        self.closed = True
        if self.peer is not None:
            self.peer.peer_closed = True


class Listener:
    def __init__(self):
        self.backlog = collections.deque()

    def accept(self):
        if not self.backlog:
            raise BlockingIOError("暂时没有新连接")
        return self.backlog.popleft()


def listen(net, port):
    lis = Listener()
    net.listeners[port] = lis
    return lis


def connect(net, port):
    if port not in net.listeners:
        raise ConnectionRefusedError(111, "Connection refused")
    cli = Endpoint("client")
    srv = Endpoint("server")
    cli.peer = srv
    srv.peer = cli
    net.listeners[port].backlog.append(srv)
    return cli


def req(*parts):
    return (" ".join(parts) + "\n").encode()


def decode(line):
    bits = line.decode().split(" ")
    return bits[0], bits[1:]


def handle(store, line):
    # PUT k v -> OK；GET k -> 值 or NONE；DEL k -> OK/NONE；别的 -> ERR
    cmd, args = decode(line)
    if cmd == "PUT" and len(args) == 2:
        store[args[0]] = args[1]
        return b"OK\n"
    if cmd == "GET" and len(args) == 1:
        return (store.get(args[0], "NONE") + "\n").encode()
    if cmd == "DEL" and len(args) == 1:
        return (b"OK\n" if store.pop(args[0], None) is not None else b"NONE\n")
    return b"ERR\n"


def send_req(cli, *parts):
    # 客户端发一条请求：用空格连、末尾加换行
    cli.sendall((" ".join(parts) + "\n").encode())


def read_reply(cli):
    # 收一行回复、去掉末尾换行、解码成字符串
    return cli.recv(4096).split(b"\n", 1)[0].decode()


def roundtrip(net, port, store, *parts):
    # 一次完整来回：客户端发、服务端 handle、客户端读回复
    cli = connect(net, port)
    srv = net.listeners[port].backlog[-1]
    send_req(cli, *parts)
    srv.sendall(handle(store, srv.recv(4096).split(b"\n", 1)[0]))
    return read_reply(cli)

net = Net()
lis = listen(net, 9000)
store = {}
roundtrip(net, 9000, store, "PUT", "k", "v")
print(roundtrip(net, 9000, store, "GET", "k"))