I'm not a professional programmer, but I think it comes down to whether you expect to ever do major refactoring. If requirements change substantially, would you

I'm not a professional programmer, but I think it comes down to whether you expect to ever do major refactoring. If requirements change substantially, would you write new code, or would you modify the existing code and attempt to maintain some kind of backward compatibility? If the latter, tests can be worth it. I have a library that never had any tests until I wanted to re-architect it in a fundamental way whilst porting it from Python 2 to Python 3. Since it was a wire-protocol type of thing, the Python bytes–unicode distinction made this hard. So I wrote tests for each piece of functionality as I ported that piece. I keep the tests around nowadays even though the porting is complete, but am kind of lazy writing ones for new functionality. Of course tests also seem really important when you're accepting 3rd party contributions to your codebase. I've made what I thought were totally innocuous pull requests to projects only to see in the CI tests that my code causes regressions.