Master Python Socket Programming: From Basics to Full TCP/UDP Client‑Server

Introduction

Socket is a communication endpoint that can connect processes on the same host or across different hosts and networks. Python’s socket module provides classes and methods that simplify socket programming, and this note records the essential concepts and usage.

Socket families and types

socket.AF_UNIX

– local inter‑process communication on Unix systems. socket.AF_INET – IPv4 network communication. socket.AF_INET6 – IPv6 network communication. socket.SOCK_STREAM – stream‑oriented socket (TCP). socket.SOCK_DGRAM – datagram socket (UDP). socket.SOCK_RAW – raw socket for low‑level protocols such as ICMP. socket.SOCK_SEQPACKET – reliable sequenced packet service.

Key socket functions

Server side

socket.bind(address)

– bind the socket to a (host, port) tuple. socket.listen(backlog) – start listening for incoming TCP connections; backlog is the maximum pending connections (minimum 1). socket.accept() – accept a pending connection and return a new socket object and the client address. socket.recv(bufsize[, flags]) – receive up to bufsize bytes from a TCP socket. socket.send(data[, flags]) – send a byte string over a TCP socket. socket.sendall(data[, flags]) – ensure the entire byte string is transmitted. socket.close() – close the socket.

Client side

socket.connect(address)

– connect to a remote (host, port) tuple; raises socket.error on failure. socket.connect_ex(address) – like connect but returns 0 on success or an error number.

Same send, recv, and close methods as the server.

Creating a TCP connection

Server steps

Create a socket: tcpSock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) Bind to an address and port: tcpSock.bind((host, port)) Listen for connections: tcpSock.listen(backlog) Accept a client: conn, addr = tcpSock.accept() Exchange data using conn.send() and conn.recv().

Close the connection with conn.close() and finally tcpSock.close().

Client steps

Create a socket with the same parameters.

Connect to the server: tcpCliSock.connect((host, port)) Send data using tcpCliSock.send() and receive responses with tcpCliSock.recv().

Terminate the session with tcpCliSock.close().

Sample implementation

Server (server.py):

#coding:utf8
import socket
from time import ctime

HOST = ''
PORT = 9999
BUFSIZE = 1024
ADDR = (HOST, PORT)

tcpSock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
tcpSock.bind(ADDR)
tcpSock.listen(5)

while True:
    print('waiting for connection ....')
    tcpCliSock, addr = tcpSock.accept()
    print('...connected from ', addr)
    while True:
        data = tcpCliSock.recv(BUFSIZE)
        if not data:
            break
        elif data == 'exit':
            tcpCliSock.send('see you again')
            tcpCliSock.close()
            exit(0)
        tcpCliSock.send('%s, %s ' % (ctime(), data))
        print('[%s]:' % ctime(), data)
    tcpCliSock.close()

tcpSock.close()

Client (client.py):

#coding:utf8
import socket

HOST = '10.0.2.6'  # server IP
PORT = 9999
BUFSIZE = 1024
ADDR = (HOST, PORT)

tcpCliSock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
tcpCliSock.connect(ADDR)

while True:
    try:
        data = raw_input('> ')
        if not data:
            break
        tcpCliSock.send(data)
        resp = tcpCliSock.recv(BUFSIZE)
        if not resp:
            break
        print('%s say: %s' % (HOST, resp))
        if resp == 'see you again':
            exit(0)
    except (KeyboardInterrupt, SystemExit):
        tcpCliSock.close()
        exit(0)

The two scripts demonstrate a minimal TCP client‑server exchange, including connection setup, message exchange, and graceful shutdown.