That's a nice rule of thumb, and like many other good rules of thumb it has an opposite: Don't even try to write code you won't be able to test. Both of those

That's a nice rule of thumb, and like many other good rules of thumb it has an opposite: Don't even try to write code you won't be able to test. Both of those have good reasons, following them makes sense, and if you're at all lucky there's no conflict.