Behind the Scenes of tox: The Journey of Rewriting a Python Tool with more than 10 Million Monthly Downloads

Do you recall what developer legend Joel Spolsky called the "single worst strategic mistake" in "Things You Should Never Do"?

Rewriting software from scratch.

That is what we did. For the tox test automation tool. A tool, downloaded more than 10 million times a month, both heavily used in the open source community, and in multi-billion dollar companies alike.

I invite you to join me on the very personal journey of rewriting tox, a journey, which already started in 2019. We will have a look at the initial motivations for the rewrite, the design decisions, the challenges, and the lessons learned.

We will reconstruct why it took more than three years, from the idea to the release, and why this was a good thing.

I will explain what we did to take care that the release would cause the least amount of issues, and why we still received dozens and dozens of bug reports about regressions the days after the release.

And finally, I will answer the question. Was it worth it?