But for those of us who work on a team, it's far more complicated than that, and you have no idea who might be touching your code in the future.
I think addresses that point quite nicely?
But the collective mistakes of a team aren't a predictor of potential future members of the team. For example, we started hiring "junior" programmers, which increases the skill gap.
I very rarely have issues in problem space A, but I don't really test for A. Maybe a few here or there but it's a small minority of my tests and the coverage is probably just incidental. I test heavily in problem space B for whatever reason.
I get hit by a bus, and you're my replacement.
You have no experience with problem space A, so there are few if any tests around that space. All of a sudden the coverage is misleading because we might have 100% coverage or close to it, but the thoroughness isn't there and suddenly releases are less reliable and support ticket volumes increase.
Even if I'm alone in my team, When I refactor code after say 10 or 15 years I could just as well have been another person. I don't remember. I don't have the same skill set.
And the thing is: when you first write the code you usually don't know how many years you will maintain it. That might be a reason to postpone some test writing a while initially (if the code is scrapped in 2 months what whas the point of effort for maintainability?)
However there are upfront benefits with writing at least some tests for everything, in that it (usually) helps design a less coupled system.
That seems to cover the team element, too. Even if I'm not having a bad day, someone else messing around in my code might be.
Not only that, but you have to be able to test against yourself a month from now or a year from now, when you've completely forgotten some aspects of the design and might overlook something doing a modification. I write a lot of report generators at my job, so there's quite often a bug or a new feature in a report I haven't touched for months at a time.
There does seem to be a lot of resistance to the idea of handing problems to solo programmers right now though. Any hints as to how you've made this work?