Python 3 Standard Library Socket Programming Cookbook: TCP, UDP, select.select(), struct Packing, and Checksums

import sys, struct, json, hashlib, zlib # 1. Command-Line Arguments (python3 script.py localhost 10000 input.txt) ip = sys.argv[1] port = int(sys.argv[2]) filename = sys.argv[3] # 2. Struct Packing & Unpacking packer = struct.Struct('i i 1s') packed_bytes = packer.pack(10, 25, b'+') a, b, raw_op = packer.unpack(packed_bytes) op = raw_op.decode().strip('\x00') # 3. Delimiter-Separated Payloads (':', ';', '|') numbers = [3, 7, 12, 18, 20] bet = 500 msg_bytes = (":".join(str(x) for x in numbers) + f":{bet}").encode() parts = [int(x) for x in msg_bytes.decode().split(":")] recv_nums, recv_bet = parts[:-1], parts[-1] # 4. Reading Text, JSON, and Binary Files with open("input.json", "r") as f: data = json.load(f) # 5. MD5 and CRC32 Checksums md5_hex = hashlib.md5(b"1:5:12:19:20").hexdigest() crc_int = zlib.crc32(b"1:5:12:19:20") 

# --- tcp_server.py --- import socket, sys ip, port = sys.argv[1], int(sys.argv[2]) with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as server: server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) server.bind((ip, port)) server.listen(5) print(f"TCP Server listening on {ip}:{port}") while True: conn, addr = server.accept() with conn: print(f"Connected by {addr}") data = conn.recv(1024) if data: print("Received:", data.decode()) conn.sendall(b"Hello client") 

# --- tcp_client.py --- import socket, sys ip, port = sys.argv[1], int(sys.argv[2]) with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as client: client.connect((ip, port)) client.sendall(b"Hello server") print("Server replied:", client.recv(1024).decode()) 

# --- select_tcp_server.py --- import socket, select, sys ip, port = sys.argv[1], int(sys.argv[2]) server = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) server.bind((ip, port)) server.listen(5) inputs = [server] print(f"Multi-client TCP Server running on {ip}:{port}...") while True: readable, _, _ = select.select(inputs, [], [], 1.0) for s in readable: if s is server: conn, addr = server.accept() print(f"[+] New client connected: {addr}") inputs.append(conn) else: data = s.recv(1024) if not data: print("[-] Client disconnected") inputs.remove(s) s.close() else: print(f"[<] Received: {data.decode()}") s.sendall(b"OK") 

# --- udp_server.py --- import socket, sys sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) sock.bind((sys.argv[1], int(sys.argv[2]))) while True: data, client_addr = sock.recvfrom(1024) print(f"Received {data.decode()} from {client_addr}") sock.sendto(b"Hello Client", client_addr) 

# --- udp_client.py --- import socket, sys server_addr = (sys.argv[1], int(sys.argv[2])) sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) sock.settimeout(2.0) try: sock.sendto(b"Hello Server", server_addr) data, _ = sock.recvfrom(1024) print("Received:", data.decode()) except (socket.timeout, ConnectionResetError): print("UDP Server did not respond!") finally: sock.close() 

# --- tcp_udp_proxy.py --- # Usage: python3 tcp_udp_proxy.py <proxy_ip> <proxy_port> <server_ip> <server_port> import socket, select, struct, sys proxy_addr = (sys.argv[1], int(sys.argv[2])) server_addr = (sys.argv[3], int(sys.argv[4])) tcp_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) tcp_sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) tcp_sock.bind(proxy_addr) tcp_sock.listen(5) udp_sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) udp_sock.settimeout(2.0) packer = struct.Struct("i i c") inputs = [tcp_sock] while True: readable, _, _ = select.select(inputs, [], []) for s in readable: if s is tcp_sock: conn, addr = tcp_sock.accept() print("Client connected to proxy:", addr) inputs.append(conn) else: msg = s.recv(packer.size) if not msg: inputs.remove(s) s.close() continue udp_sock.sendto(msg, server_addr) result, _ = udp_sock.recvfrom(packer.size) val, zero, r_char = packer.unpack(result) s.sendall(packer.pack(val * 2, zero, r_char)) 

Dune;Hall 1;17:30;5 Oppenheimer;Hall 2;19:00;3 Barbie;Hall 1;20:45;0 

# --- reservation_server.py --- # Usage: python3 reservation_server.py <ip> <port> <screenings.txt> import socket, select, struct, sys, random, string ip, port, filename = sys.argv[1], int(sys.argv[2]), sys.argv[3] screenings = {} with open(filename, "r") as f: for line in f: line = line.strip() if line: title, hall, start_time, seats = line.split(";") screenings[f"{title};{start_time}"] = { "title": title, "hall": hall, "time": start_time, "seats": int(seats) } packer = struct.Struct("7s6s") server = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) server.bind((ip, port)) server.listen(5) inputs = [server] print(f"Server running on {ip}:{port}...") while True: readable, _, _ = select.select(inputs, [], []) for s in readable: if s is server: conn, addr = server.accept() print(f"[+] Client connected: {addr}") inputs.append(conn) else: data = s.recv(1024) if not data: print("[-] Client disconnected") inputs.remove(s) s.close() continue msg = data.decode().strip() print(f"[<] Received: {msg}") if msg == "GET_SCREENINGS": reply_str = "|".join( f"{v['title']};{v['hall']};{v['time']}" for v in screenings.values() ) s.sendall(reply_str.encode()) print(f"[>] Sent screenings: {reply_str}") else: if msg in screenings and screenings[msg]["seats"] > 0: screenings[msg]["seats"] -= 1 ticket_id = "".join(random.choices(string.ascii_uppercase + string.digits, k=6)) s.sendall(packer.pack(b"SUCCESS", ticket_id.encode())) print(f"[>] SUCCESS for {msg}. ID: {ticket_id} (Seats left: {screenings[msg]['seats']})") else: s.sendall(packer.pack(b"FAIL", b"000000")) print(f"[>] FAIL for {msg}.") 

# --- reservation_client.py --- # Usage: python3 reservation_client.py <ip> <port> <film_title> <start_time> import socket, struct, sys ip, port = sys.argv[1], int(sys.argv[2]) film = sys.argv[3] if len(sys.argv) > 3 else "Dune" time_str = sys.argv[4] if len(sys.argv) > 4 else "17:30" packer = struct.Struct("7s6s") with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as client: client.connect((ip, port)) client.sendall(b"GET_SCREENINGS") print("[>] Sent: GET_SCREENINGS") screenings_raw = client.recv(1024).decode() print("[<] Available screenings:") for item in screenings_raw.split("|"): print(" -", item) buy_req = f"{film};{time_str}" client.sendall(buy_req.encode()) print(f"[>] Attempting to reserve: {buy_req}") resp_data = client.recv(packer.size) status_bytes, ticket_bytes = packer.unpack(resp_data) status = status_bytes.decode().strip('\x00') ticket_id = ticket_bytes.decode().strip('\x00') print(f"[<] Server Result: {status}, Ticket ID: {ticket_id}") 

# --- intercept_proxy.py --- # Usage: python3 intercept_proxy.py <proxy_ip> <proxy_port> <server_ip> <server_port> import socket, struct, sys, random proxy_addr = (sys.argv[1], int(sys.argv[2])) server_addr = (sys.argv[3], int(sys.argv[4])Source：https://dev.to/unnobatroo/python-3-standard-library-socket-programming-cookbook-tcp-udp-selectselect-struct-packing-4hjj
My favorites