基于select模式启动服务器，然后启动两个客户端 A 和 B，客户端 A 发消息给服务器，服务器转发给客户端 B ，客户端B打印显示接收到的消息。客户端B发消息给服务器，服务器转发给客户端A，客户端A打印显示接收到的消息。

以下是基于select模式实现的服务器和客户端代码：

服务器代码：

import socket
import select

server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
server_socket.bind(('localhost', 8888))
server_socket.listen(5)

sockets_list = [server_socket]
clients = {}

print('Server started and listening on port 8888')

def receive_message(client_socket):
    try:
        message_header = client_socket.recv(1024)
        if not len(message_header):
            return False
        message_length = int(message_header.decode('utf-8').strip())
        return {'header': message_header, 'data': client_socket.recv(message_length)}
    except:
        return False

while True:
    read_sockets, _, exception_sockets = select.select(sockets_list, [], sockets_list)

    for notified_socket in read_sockets:
        if notified_socket == server_socket:
            client_socket, client_address = server_socket.accept()
            user = receive_message(client_socket)
            if user is False:
                continue
            sockets_list.append(client_socket)
            clients[client_socket] = user
            print(f'Accepted new connection from {client_address[0]}:{client_address[1]} '
                  f'username:{user["data"].decode("utf-8")}')
        else:
            message = receive_message(notified_socket)
            if message is False:
                print(f'Closed connection from {clients[notified_socket]["data"].decode("utf-8")}')
                sockets_list.remove(notified_socket)
                del clients[notified_socket]
                continue
            user = clients[notified_socket]
            print(f'Received message from {user["data"].decode("utf-8")}: {message["data"].decode("utf-8")}')
            for client_socket in clients:
                if client_socket != notified_socket:
                    client_socket.send(user['header'] + user['data'] + message['header'] + message['data'])
    
    for notified_socket in exception_sockets:
        sockets_list.remove(notified_socket)
        del clients[notified_socket]

客户端A代码：

import socket
import sys

client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client_socket.connect(('localhost', 8888))

username = input('Enter your username: ')
client_socket.send(bytes(username, 'utf-8'))

while True:
    message = input(f'{username} > ')

    if message:
        message = bytes(message, 'utf-8')
        message_header = f'{len(message):<10}'.encode('utf-8')
        client_socket.send(message_header + message)

    try:
        while True:
            username_header = client_socket.recv(10)
            if not len(username_header):
                print('Connection closed by the server')
                sys.exit()
            username_length = int(username_header.decode('utf-8').strip())
            username = client_socket.recv(username_length).decode('utf-8')
            message_header = client_socket.recv(10)
            message_length = int(message_header.decode('utf-8').strip())
            message = client_socket.recv(message_length).decode('utf-8')
            print(f'{username} > {message}')
    except:
        continue

客户端B代码：

import socket
import sys

client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client_socket.connect(('localhost', 8888))

username = input('Enter your username: ')
client_socket.send(bytes(username, 'utf-8'))

while True:
    try:
        while True:
            username_header = client_socket.recv(10)
            if not len(username_header):
                print('Connection closed by the server')
                sys.exit()
            username_length = int(username_header.decode('utf-8').strip())
            username = client_socket.recv(username_length).decode('utf-8')
            message_header = client_socket.recv(10)
            message_length = int(message_header.decode('utf-8').strip())
            message = client_socket.recv(message_length).decode('utf-8')
            print(f'{username} > {message}')
    except:
        continue

    message = input(f'{username} > ')

    if message:
        message = bytes(message, 'utf-8')
        message_header = f'{len(message):<10}'.encode('utf-8')
        client_socket.send(message_header + message)