G'day
A coupla days ago someone shared this Twitter status with me:
I'll just repeat the text here for google & copy-n-paste-ability:
My personal opinion: Having 100% test coverage doesn't make your code more effective. Tests should add value but at some point being overly redundant and testing absolutely every line of code is ineffective. Ex) Testing that a component renders doesn't IN MY OPINION add value.
Emma's position here is spot on, IMO. There were also a lot of "interesting" replies to it. Quelle surprise. Go read some of them, but then give up when they get too repetitive / unhelpful.
In a similar - but slightly contrary - context, I was reminded of my erstwhile article on this topic: "Yeah, you do want 100% test coverage".
But hang on: on one hand I say Emma is spot on. On the other hand I cross-post an old article that seems to contradict this. What gives?
The point of differentiation is "testing absolutely every line of code" vs "100% test coverage". Emma is talking about lines of code. I'm talking about test cases.
Don't worry about testing lines of code. That's intrinsically testing "implementation". However do care about your test cases: the variations that define the feature you are working on. you've been asked to develop a feature and its variations, and you don't know you've delivered the feature (or in the case of automated testing: continue to having been delivered in a working state) unless you test it.
Now, yeah, sure: features are made of lines of code, but don't worry about those. A trite example I posited to a mate yesterday is this: consider this to be the implementation of your feature:
doMyFeature() {
// line 1
// line 2
// line 3
// line 4
// line 5
// line 6
// line 7
// line 8
// line 9
// line 10
}
I challenge you to write a test for "My Feature", and not by implictation happen to test all those lines of code. But it's the feature we care about, not the lines of code.
On the other hand, let's consider a variant:
doMyFeature() {
// line 1
// line 2
// line 3
// line 4
// line 5
if (someVariantOfMyFeature) {
// line 6
}
// line 7
// line 8
// line 9
// line 10
}
If you run your test coverage analysis and see that line 6 ain't tested, this is not a case of wringing one's hands about line 6 of the code not being tested; it's that yer not testing the variation of the feature. It's the feature coverage that's missing. Not the lines-of-code coverage.
Intrinsically your code coverage analysis tooling probably marks individual lines of code as green or red or whatever, but only if you look that closely. If you zoom out a bit, you'll see that the method the lines of code are in is either green or orange or red; and out further the class is likewise green / orange / red, and probably says something like "76% coverage". The tooling necessarily needs to work in lines-of-code because its a dumb machine, and those are the units it can work on. You are the programmer don't need to focus on the lines-of-code. You have a brain, and what the report is saying is "you've not tested part of your feature", and it's saying "the bit of the feature that is represented by these lines of code". That bit. You're not testing it. Go test yer feature properly.
There's parts of the implementation of a feature I won't test maybe. Your DAOs and your other adapters for external services? Maybe not something I'd test during the TDD cycle of things, but it might still be prudent to chuck in an integration test for those. I mean they do need to work and continue to work. But I see this as possibly outside of feature testing. Maybe? Is it?
Also - now I won't repeat too much of that other article - there's a difference between "coverage" and "actually tested". Responsible devs can mark some code as "won't test" (eg in PHPUnit with a @codeCoverageIgnore), and if the reasons are legit then they're "covered" there.
Why I'm writing this fairly short article when it's kinda treading on ground I've already covered is because of some of the comments I saw in reply to Emma. Some devs are a bit shit, and a bit lazy, and they will see what Emma has written, and their take away will be "basically I don't need to test x because x is some lines of code, and we should not be focusing on lines of code for testing". That sounds daft, but I know people that have rationalised their way out of testing perfectly testable (and test-necessary) code due to "oh we should not test implementation details", or "you're just focusing on lines of code, that's wrong", and that sort of shit. Sorry pal, but a feature necessarily has an implementation, and that's made out of lines of code, so you will need to address that implementation in yer testing, which will innately report that those lines of code are "covered".
Also this notion of "100% coverage is too high, so maybe 80% is fine" (possibly using the Pareto Pinciple, poss just pulling a number of out their arses). Serious question: which 80%? If you set that goal, the a lot of devs will go "aah, it's in that 20% we don't test". Or they'll test the easy 80%, and leave the hard 20% - the bit that needs testing - to be the 20% they don't test (as I said in the other article: cos they basically can't be arsed).
Sorry to be a bit of a downer when it comes to devs' level of responsibility / professionalism / what-have-you. There are stack of devs out there who rock and just "get" what Emma (et al) is saying, and crack on with writing excellently-designed and well-tested features. But they probably didn't need Emma's messaging in the first place. I will just always think some more about advice like this, and wonder how it can be interpreted, and then be a contrarian and say "well actually…" (cringe, OK I hope I'm not actually doing that here?), and offer a different informed observation.
So my position remains: set your sights on 100% feature coverage. Use TDD, and your code will be better and leaner and you'll also likely end up with 100% LOC coverage too (but that doesn't matter; it's just a test-implementation detail ;-).
Righto.
--
Adam
