Simon Willison’s Weblog

How to cheat at unit tests with pytest and Black

11th February 2020

I’ve been making a lot of progress on Datasette Cloud this week. As an application that provides private hosted Datasette instances (initially targeted at data journalists and newsrooms) the majority of the code I’ve written deals with permissions: allowing people to form teams, invite team members, promote and demote team administrators and suchlike.

The one thing I’ve learned about permissions code over the years is that it absolutely warrants comprehensive unit tests. This is not code that can afford to have dumb bugs, or regressions caused by future development!

I’ve become a big proponent of pytest over the past two years, but this is the first Django project that I’ve built using pytest from day one as opposed to relying on the Django test runner. It’s been a great opportunity to try out pytest-django, and I’m really impressed with it. It maintains my favourite things about Django’s test framework—smart usage of database transactions to reset the database and a handy test client object for sending fake HTTP requests—and adds all of that pytest magic that I’ve grown to love.

It also means I get to use my favourite trick for productively writing unit tests: the combination of pytest and Black, the “uncompromising Python code formatter”.

Datasette and sqlite-utils, and those are by far the best tested pieces of software I’ve ever released.

I started doing this around two years ago, and I’ve held off writing about it until I was confident I understood the downsides. I haven’t found any yet: I end up with a robust, comprehensive test suite and it takes me less than half the time to write the tests than if I’d been hand-crafting all of those comparisons from scratch.

• datasette-auth-existing-cookies 0.6 and 0.6.1
• Shipped sqlite-utils 2.2, 2.2.1, 2.3 and 2.3.1
• Shipped Datasette 0.35 with a new utility method for plugins to render their own templates, which I’m now using in…
• datasette-upload-csvs 0.2a—still very alpha, but at least it looks slightly nicer now

Unrelated to Datasette Cloud, I also shipped twitter-to-sqlite 0.16 with a new command for importing your Twitter friends (previously it only had a command for importing your followers).

In bad personal motivation news… I missed my weekly update to Niche Museums and lost my streak!

Part of series My open source process

1. Documentation unit tests - July 28, 2018, 3:59 p.m.
2. How to cheat at unit tests with pytest and Black - Feb. 11, 2020, 6:56 a.m.
3. Open source projects: consider running office hours - Feb. 19, 2021, 9:54 p.m.
4. How to build, test and publish an open source Python library - Nov. 4, 2021, 10:02 p.m.
5. How I build a feature - Jan. 12, 2022, 6:10 p.m.
6. Writing better release notes - Jan. 31, 2022, 8:13 p.m.
7. Software engineering practices - Oct. 1, 2022, 3:56 p.m.
8. … more