[simonw]

> You can test every line of code in your program, but how many people actually do?

In my mind, that's what separates genuinely excellent professional programmers from everybody else.

[HarHarVeryFunny]

I think it's perhaps more that you learn to write code that is easy to test and debug, consisting of some minimal set of simple orthogonal components, etc. You test every function of course, but learn to intuitively design out these combinatorial complexities that could nonetheless still be lurking, and pre-emptively include assertions to try to catch anything you may have overlooked.