Test-driven Generation: A Hands-on Experience

for EuroSTAR2025, Session DD1 at 10:30 on Weds 3 June

Repo for code etc. you used in the session: GenFromTestsTrial

Repo for environments (not up to date with what we used): buildServerForTestingAndCodingExercise

Welcome to the deep dive! In this hands-on session, we'll write tests that fail, and an LLM will hand back code which passes those tests.

We will be using a Miro board to share ideas. There's space on that board to share what you're hoping to get from the session, to work with your group, and to share at the end.

Here are two pictures: what the tiny system to generate code from tests is doing, and what goes into the prompt to send to the LLM. We'll refer back to these.

And a picture of what gets sent to the LLM

At the end

Fair warning: we'll spend 10-15 minutes at the end thinking and sharing. Work towards this by putting thoughts onto the Miro board as we go.

Setup

You're sat at tables. Each table has a "Group": group01 to group12.

Each group has its own server, at url groupxx.workroomprds.com. Here are the links: group01, group02, group03, group04, group05, group06, group07, group08, group09, group10, group11, group12.

Please open your own group's URL, pick a (unique) user from the list and follow the link. That is your development environment – your password is password.

If you're early and feel the urge to influence, put what you want on the Miro board.

Exercise 0: Demo

We'll start with a demo. We'll do it twice. First time you watch, second time you do it yourself. You'll want to be in the "code server" environment, in the initial_python directory. I'll show you what that means.

In the demo, I'll give a set of tests to an LLM, and ask it to code a festival.py thing that can produce all those dates. I'll start with no code, and see what it makes.

I'll open my code-server development environment to get access to a directory browser, file editor and commandline.

First I'll source ~/llm-env/bin/activate to start the tool that talks with LLMs. I'll check my commandline starts with (llm-env).

I'll use ./makeNewPythonFromTests.sh festival.py to generate some code.

We'll use the file editor to watch the output and look at the code. Then we'll bin the code, do it again, and compare using change control.

Exercise 1: Repeat Demo

Follow me again, but this time take actions:

Open your link to code-server (which is VSCode in the browser so should be familiar). Set it up by closing first-time dialogs and opening the terminal to use the commandline. Open the code directory in the directory browser.

Use cd to change the terminal's directory to code/initial_python

Remember to source ~/llm-env/bin/activate

Use ./makeNewPythonFromTests.sh festival.py

Use the directory browser to open src/festival.py, and watch it build. Watch the progress of the tool in the terminal.

I hope our experiences will be similar, but the code will be different. We'll talk about that.

Let's play: maybe you want to add a test, or change a test, or use a new set of tests.

Maybe read tests/test_oddEven.py for different tests,
then try./makeNewPythonFromTests.sh oddEven.py .

While you're playing, use these to run the tests (and see the coverage):

• pytest ./tests/test_oddEven.py --cov=src -v (if we're playing with it)
• pytest ./tests/test_festival.py --cov=src -v

Use change control to see the differences.

Use python3 ./main.py to play with these two as working bits of software.

Share your insights and perspective changes on the Miro

Pause

It's time to talk about how this works. We'll use the diagrams above to help understand what's being sent to the LLM for it to do its non-determinstic work, and how that works with the tools.

Exercise 2: Making Code

Let's switch to something bigger: a multi-file application which takes configuration and serves web pages; Python via Flask and JavaScript via Nginx.

Play with the applications by going to your development environment entry and picking rs_py or rs_js App. They should be very similar – they pass broadly the same tests.

Those tests are in three parts – a setup part which introduces a test scale, a part which tests the conversion, and a part which tests the compatibility.

Decide, as a group, whether you want to work on the Python / Pytest one, or the JavaScript / Jest one.

For crispness, deactivate your Python virtual environment with deactivate.

The code is in Python (rs_py) or JavaScript (rs_js). You'll want to navigate your terminal with cd ../rs_js or cd ../rs_py and navigate your file explorer by mousework. Reactivate your llm devenv when you get there.

Python / Pytest

Your tests are in ~/code/rs_py/tests/test_relative_sizes.py

Read the tests, and identify the parts.

The tool has made the code in ~/code/rs_py/tests/test_relative_sizes.py, and you have already been using it.

To re-make the code, run ./makeNewPythonFromTests.sh relative_sizes.py

The code does not pass the tests. The script will try to make them pass, and may fail, or may succeed.

As a group, as a pair, or solo, decide what you'll do – change the tests, change the code, delete the code and remake from scratch.

When the LLM has delivered new code that passes the tests, it is checked in. However, you need to restart flask to pick it up in the web interface:

sudo systemctl restart flask-«your CLI ID»-rs_py

Share your insights and perspective changes on the Miro

JavaScript / Jest

Your tests are in ~/code/rs_js/test/jest/relativeSizes.test.js

Read the tests, and identify the parts.

The tool has made the code in ~/code/rs_js/src/js/relativeSizes.js, and you've already been using it.

To re-make the code, run ./makeNewJSFromTests.sh relativeSizes.js

The script should output that the code, having passed the tests, doesn't need to be re-made. So you could delete the code, break the code, or add new (failing) tests.

Reload your browser window from origin to pick up the new code.

Share your insights and perspective changes on the Miro

Exercise 2: Exploring the weirdness

Decide as a group what you might explore. Some starting suggestions:

• Make code several times and see what repeats
• Refactor the tests
• Increase the functionality by adding tests
• Change the functionality by changing tests
• Give the script conflicting tests
• Change the scales
• Explore via the interface
• Compare Python and JS approaches
• Try different LLMs (you've got access to all of Anththropic and OpenAI – 4o-mini is interesting)
• Try an different architecture
• Try generating a different part
• Try changing the order of the tests
• Try changing the names of the tests
• Try changing rules.md (in JS)

Decide publicly what you'll play with. Volunteer information to the whole room about what you found.

Share your insights and perspective changes on the Miro

WRAPUP

Last 15 minutes

• put more insights on the miro board
• talk, work out what you want to say to the room
• say it.

Tools and command-line reference

Code-Server IDE

It's VSCode in the browser – menu options are under the three bars top-left, directory browser is the stack of paper, search is the magnifying glass.

Passwords are all password

It's pretty tab-happy (within your browser tab). If you need independent windows you need to open those in a private tab so they dont interfere with each other.

If you can’t go “up” in the directory / can’t see the top of a tree, use menu: file: open folder.

Open the Terminal

To open  the terminal: look to top-right, open the panel,

then select the terminal

Please note – your access is not sandboxed: you can go into other users’ home

Working with change control

It's git. Select the change control icon on the LHS.

Commandline

source ~/llm-env/bin/activate to activate the llm tool and its python virtual environment. You should see (llm-env) at the start of your commandline when this is working. Deactivate it with deactivate

Change the prompt?

Use llm to change a prompt or write a new one

Change your script to pick up a new prompt.

Working with llm

Work on the command line with commands like:
llm --help
llm templates --help
llm templates list
llm templates show «template name»

Changing a prompt
llm templates edit «template name»
Or you can go to llm templates path and edit the file in the editor.
llm keys «your provider» to give it your API key
llm install «plugin for provider» to access a different LLM (you’ll need a key)
llm models to list models

Working with the script

Change MAX_ATTEMPTS to give the LLM more attempts within the same conversation
Change LLM_TEMPLATE to pick up your own prompt
Change LLM_MODEL to switch model

Working with the sources

Change the code to pass a test – it’ll be an input to the next round, but may not stick!
Change the tests to push the LLM towards generating different code.
Change rules.md (or add comments to the tests) to give language-based hints

Running the tests

Jest

NODE_OPTIONS=--experimental-vm-modules npx jest --testPathPattern=web/test/jest/htmlhandler.test.js --collectCoverageFrom=web/src/js/htmlhandler.js

Python

pytest tests/test_relative_sizes.py -v

Troubleshooting web stuff

Python

App page gives a 502 error? Perhaps you’ve got a bad gateway?

sudo systemctl status flask-james-webpy01 to see if flask is running (if it’s not, there’s no web page)

sudo journalctl -u flask-james-webpy01.service -f --no-pager -n 50 to see what’s up with flask

If you see ImportError: cannot import name  you may have an integration error.

Check Flask service sudo systemctl status flask-«james»-«webpy01»

If it's failed, check why sudo journalctl -u flask-«james»-«webpy01» -n 20

Check if port is listening (see bottom of IDE window for port) sudo netstat -tlnp | grep :«8001»

Check nginx error logs sudo tail -n 20 /var/log/nginx/error.log

More

Here are some insights into the choice of LLM

Different LLMs do Different Things

Can I substitute GPT-4, 4o-mini or Qwen3 for Claude3.5Sonnet? tl;dr: Nope.

Workroom ProductionsJames Lyndsay

Here's what the workshop cost

Workshop Costs

Delivery costs for running a large tech workshop

Workroom ProductionsJames Lyndsay