I thought about that, but decided against it because it would be confusing for new users. And I don’t think it’s all that terrible if tests are loaded in production code, anyway.

Doctest has another interesting property, in that you can write documentation in a straightforward manner, and it just so happens that documentation is a test. Which means your doc’s sample code is going to be correct and working, and when those tests fail you’ll know to update the documentation.

A practice every language and library should adopt.

That’s how it’s intended to be used, but in practice, I’ve mostly seen them stored separately. Personally, I think that’s because tests are a helpful supplement to documentation, but often aren’t clear documentation by themselves.