Lab 4: Python SocketsSpring 2026

Introduction

This lab seeks to help introduce you to TCP sockets, and how to use them in Python since you will be doing this in the networking project. You will write a script that connects to a server, answers a few addition questions, and then gets a secret code.

Setup

If you haven’t already, follow our Docker guide to learn how to set up Docker on your computer. This will be useful both for this lab as well as for the remaining projects and labs.

To get the code for this lab, create a repo using the GitHub template. Make sure that this repository is private. Clone the repository onto your system, then open it in VS Code. If you successfully set up Docker, you should be greeted with a pop-up in the bottom right asking you to reopen the directory in the development container; do so now.

Resources

Python Socket Basics

• Python Socket Programming HOWTO
• Python socket Module

Background

You already have experience with HTTP, which is a protocol for communicating over the network. It is convenient, but it’s also quite limited. The only communication scheme it allows is for the client to send a complete message, and then for the server to respond with a complete message.

TCP is a lower-level protocol, which makes it a bit harder to work with, but a lot more flexible. At any moment, the client or the server can send the other a stream of bytes. This stream is guaranteed to arrive in order, but it may not arrive all at once. This allows the recipient to get started processing the data before the sender has even finished sending it.

Task

Overview

You will write a Python script that interacts with a server over TCP. The server will ask your script a few simple addition questions, one at a time, and wait for your answers before sending the next one. If you answer them all correctly, it will give you a secret phrase that you will print out.

It may be helpful to reference the Python Socket Programming HOWTO guide and the Python socket module documentation while working on this lab.

Running the script

Your script should take 2 command line arguments, which tell your script how to locate the server. The first one is the hostname. The second one is the port number. When running your script locally, use mathserver and 8000. You have to be inside the Docker container in order to connect to the mathserver.

python3 lab4.py mathserver 8000

You can also run your script in the VS Code debugger.

Connecting to the Server

In order to use sockets in Python, you will have to import the socket module:

import socket

Construct a new socket:

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:

Then connect the socket to the server:

sock.connect((HOST, PORT))

Communicating with the Server

Questions & Answers

As soon as you connect to the server, it will send you your first addition question.

To receive the data it has sent you:

data = sock.recv(4096)

The question will be formatted as the following 9 bytes:

+---+---------------------------+---------------------------+
| Q | 4-byte big-endian integer | 4-byte big-endian integer |
+---+---------------------------+---------------------------+

The Q byte indicates that this is a question. You will need to convert into two integers. Add these integers together, then convert the sum into 4 bytes (using big-endianness).

Now that you have your answer in the form of bytes, send it back to the server:

sock.sendall(answer)

If you get the answer wrong, the server will close the connection early.

This question-and-answer process will repeat a random number of times. Once it’s done, the server will send you a secret phrase instead of a question.

Secret Phrase

Receiving the secret phrase is exactly the same process as receiving a question:

data = sock.recv(4096)

The secret phrase will be formatted as the following 9 bytes:

+---+----------------------+
| S | 8-byte secret phrase |
+---+----------------------+

You can tell that this is a secret phrase and not a question because it begins with an S byte instead of a Q byte.

Print this 8-byte secret phrase.

Closing the Connection

If you put the socket inside a with-statement, it will automatically be closed as soon as you leave.

If you didn’t put the socket inside a with-statement, you can close it manually with:

sock.close()

Submission

Submit the following files to the Autograder by the deadline: Thursday, March 31 at 11:59 p.m.

Web Lab Autograder