I sometimes use unit tests to fill in gaps. I just caught a function with almost full coverage except for an if statement.

Playwright e2e tests are harder to maintain than pytest code, so I wrote a test just for that one case, and while I was at it, a few other input strings that seemed like they could be problems.

If the function had no test coverage at all, I probably would have tried to get it with an integration test, because that would mean the callers also weren't getting tested.