Simon Willison’s Weblog

TIL: Subtests in pytest 9.0.0+. I spotted an interesting new feature in the release notes for pytest 9.0.0: subtests.

I'm a big user of the pytest.mark.parametrize decorator - see Documentation unit tests from 2018 - so I thought it would be interesting to try out subtests and see if they're a useful alternative.

Short version: this parameterized test:

@pytest.mark.parametrize("setting", app.SETTINGS)
def test_settings_are_documented(settings_headings, setting):
    assert setting.name in settings_headings

Becomes this using subtests instead:

def test_settings_are_documented(settings_headings, subtests):
    for setting in app.SETTINGS:
        with subtests.test(setting=setting.name):
            assert setting.name in settings_headings

Why is this better? Two reasons:

1. It appears to run a bit faster
2. Subtests can be created programatically after running some setup code first

I had Claude Code port several tests to the new pattern. I like it.