> And then, if you've already thought more complicated cases (eg negative numbers, cases that overflow) then your code already very likely handles them. And if

> And then, if you've already thought more complicated cases (eg negative numbers, cases that overflow) then your code already very likely handles them. And if you hadn't thought about them, your tests won't cover them either. Good tests check boundary conditions and edge cases. They check errors conditions (like overflow), not just the happy path. And yes, they can be a pain to write, and they affect the way your code is written. On the other hand, a test suite with good coverage can give you tremendous confidence to make sweeping changes. I have a side project, an art project, and part of that is a custom on-the-wire networking protocol. I have tests that check all the edge cases I could think of. Did I miss something? Very probably. On the other hand, I've written the protocol 3 times now: once in python, once in C, and once in C++ (the C++ case was more like a giant re-factor of the C code to make it more usable), and I'll probably write it again in rust. Each time I had confidence I got it right because my tests pass.