Comment by kazinator

The problem with TDD is that the methodology wants to cover every change, no matter how internal, with some sort of external test.

Some changes are simply not testable, period.

No, you cannot always write a test which initially fails, and then passes when the change is made, and when this is the case. You should understand why that is, and not try.

In some cases when you can, yet still should not. If a whole module is rewritten such that the new version satisfies all of the public contracts with the rest of the code, then only those contracts need to be retested; we don't need new tests targeting internals.

It's because the old version wasn't targeted by such tests in the first place that it can be rewritten without upheaval.