请使用python及socket设计程序实现ftp服务器，和编写实现的客户端进行通讯,并实现文件的上传和下载设计成多线程循环处理的模式

好的，以下是 Python 实现 FTP 服务器和客户端的代码示例：

FTP 服务器端代码：

import socket
import threading
import os

SERVER_HOST = '127.0.0.1'
SERVER_PORT = 8000
BUFFER_SIZE = 1024

class FTPServer:
    def __init__(self):
        self.server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.server_socket.bind((SERVER_HOST, SERVER_PORT))
        self.server_socket.listen(5)
        print(f'Server listening on port {SERVER_PORT}')

    def start(self):
        while True:
            client_socket, client_address = self.server_socket.accept()
            print(f'Accepted connection from {client_address}')
            t = threading.Thread(target=self.handle_client, args=(client_socket,))
            t.start()

    def handle_client(self, client_socket):
        while True:
            # receive command from client
            command = client_socket.recv(BUFFER_SIZE).decode()
            if not command:
                break

            # process command
            if command.startswith('upload'):
                self.handle_upload(client_socket, command.split(' ')[1])
            elif command.startswith('download'):
                self.handle_download(client_socket, command.split(' ')[1])
            else:
                client_socket.send(b'Invalid command\n')

        client_socket.close()

    def handle_upload(self, client_socket, filename):
        # receive file from client
        client_socket.send(b'Ready to receive file\n')
        with open(filename, 'wb') as f:
            while True:
                data = client_socket.recv(BUFFER_SIZE)
                if not data:
                    break
                f.write(data)
        client_socket.send(b'File uploaded successfully\n')

    def handle_download(self, client_socket, filename):
        # check if file exists
        if not os.path.isfile(filename):
            client_socket.send(b'File not found\n')
            return

        # send file to client
        client_socket.send(b'Ready to send file\n')
        with open(filename, 'rb') as f:
            while True:
                data = f.read(BUFFER_SIZE)
                if not data:
                    break
                client_socket.send(data)
        client_socket.send(b'File downloaded successfully\n')

if __name__ == '__main__':
    server = FTPServer()
    server.start()

FTP 客户端代码：

import socket
import os
import sys

SERVER_HOST = '127.0.0.1'
SERVER_PORT = 8000
BUFFER_SIZE = 1024

class FTPClient:
    def __init__(self):
        self.client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.client_socket.connect((SERVER_HOST, SERVER_PORT))

    def start(self):
        while True:
            # get user input
            command = input('Enter command: ')

            # send command to server
            self.client_socket.send(command.encode())

            # process command
            if command.startswith('upload'):
                self.handle_upload(command.split(' ')[1])
            elif command.startswith('download'):
                self.handle_download(command.split(' ')[1])
            elif command == 'quit':
                break
            else:
                response = self.client_socket.recv(BUFFER_SIZE)
                print(response.decode())

        self.client_socket.close()

    def handle_upload(self, filename):
        # check if file exists
        if not os.path.isfile(filename):
            print('File not found')
            return

        # send file to server
        self.client_socket.recv(BUFFER_SIZE)
        with open(filename, 'rb') as f:
            while True:
                data = f.read(BUFFER_SIZE)
                if not data:
                    break
                self.client_socket.send(data)
        response = self.client_socket.recv(BUFFER_SIZE)
        print(response.decode())

    def handle_download(self, filename):
        # receive file from server
        self.client_socket.recv(BUFFER_SIZE)
        with open(filename, 'wb') as f:
            while True:
                data = self.client_socket.recv(BUFFER_SIZE)
                if not data:
                    break
                f.write(data)
        response = self.client_socket.recv(BUFFER_SIZE)
        print(response.decode())

if __name__ == '__main__':
    client = FTPClient()
    client.start()

这里使用了多线程循环处理的模式，每个客户端连接都会启动一个新的线程来处理。FTP 服务器支持上传和下载文件，FTP 客户端可以通过输入命令来上传和下载文件。注意，此示例程序只是一个简单的实现，没有考虑安全性和错误处理等方面的问题。