Networking

Python provides low-level and high-level networking modules.

• Low-level: socket for TCP/UDP clients and servers
• High-level: urllib, http.client, smtplib, imaplib, ftplib, and others

Start with low-level sockets to understand the basics, then use higher-level libraries for production applications.

What is Socket?

A socket is an endpoint for two-way communication between programs. Sockets can communicate between processes on the same machine or across different machines on a network.

Socket Module

Create a socket with:

socket.socket(family=AF_INET, type=SOCK_STREAM)

• AF_INET for IPv4
• SOCK_STREAM for TCP
• SOCK_DGRAM for UDP

SOCK_STREAM (TCP) is reliable and ordered. SOCK_DGRAM (UDP) is faster but does not guarantee delivery.

Server Socket Methods

Client Socket Methods

General Socket Methods

Simple TCP Server

This minimal server accepts one client connection, sends a response once, and exits.

# non-runnable: requires external environment/setup
import socket

HOST = "127.0.0.1"
PORT = 12345

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen()
    print(f"Server listening on {HOST}:{PORT}")

    conn, addr = s.accept()
    with conn:
        print("Connected by", addr)
        conn.sendall(b"Thank you for connecting")

Simple TCP Client

# non-runnable: requires external environment/setup
import socket

HOST = "127.0.0.1"
PORT = 12345

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.settimeout(5)  # seconds
    s.connect((HOST, PORT))
    data = s.recv(1024)

print(data.decode("utf-8"))

Run server first, then run client from another terminal.

Common Python Internet Modules

For production systems, prefer higher-level libraries when appropriate and always add timeout handling.

Common Errors

• ConnectionRefusedError: server is not running or wrong host/port.
• Client hangs on recv(): no timeout configured and server sends no data.
• OSError: [Errno 98] Address already in use: previous process still holds the port.
• Mixing str and bytes: sockets send/receive bytes, so encode/decode explicitly.