Veit's Blog

Coverage-centric Testing


Many of my clients keep test coverage scores for every commit or pull request, and they integrate appropriate tools—such as codecov—into their code review. Coverage scores are often used as one of the primary indicators on the quality of a pull request, some even go as far as flagging a pull request as generally unfit for merging when it decreases code coverage.

I generally agree with this practice, but it encourages a set of behaviors that I think are detrimental to code quality. Today I want to talk about these.

Why does coverage decrease?

If a pull request comes in that decreases coverage, the first question I’d like to ask myself is: why does it do that? I tend to write tests while writing the code itself—I’m not always completely adherent to TDD, especially when I’m fleshing out a fresh idea—, so the coverage should at least stay the same, if not increase. If it doesn’t, that’s a bug in my workflow1.

Instead of scrambling to write some tests after the fact, I’d like to think that I look at what’s wrong with my PR first. I don’t always do that, but I try to be as conscious about it as I can.

After analyzing, I will spend some time writing tests. They’ll not be of the same quality as the tests I would’ve written if I had written them during the development of the feature, though, because the mindset is mostly to keep my pull request in the green. Not something I’m proud of, but more often the case than I’d like to admit. Although this is obviously wrong, the people who who review my code rarely object. I believe this is because this behavior is not as easy to spot as regular anti-patterns or simple bugs and often gets lost in the noise.

This brings me to the main point of my blog post: coverage-centrism.


I’ve seen people not write tests for code, because the lines introduced were already covered, and so they didn’t have to add anything. The pull request is approved, and the feature is merged.

Note that I said “lines” above, not feature. This is not the same thing at all, and even though coverage tools will report that all is well, we just missed to test a new case. Consider the following Python code2:

def my_request_handler(request):
    return HTTP_OK
Fig. 1: A useful web request.

This is a simple method using a fantasy web framework. It will return an HTTP code of 200 on every incoming request. That’s easy to test, we just call the method with a mock request and assert that we always get the expected status code back.

def test_my_request_handler():
    response = my_request_handler(mock_request())
    assert(response.status, 200)
Fig. 2: Testing our request handler.

Figure 2 is already imperfect, because we only test the function with one request. But let’s roll with it for now.

Imagine then that someone decides that we should ratelimit some endpoints of our API, and my_request_handler is one of them. We introduce a new decorator, rate_limit, that automatically does that for us.

@rate_limit(num_requests=10, seconds=60)
def my_request_handler(request):
    return HTTP_OK
Fig. 3: Adding a ratelimiter.

Although we certainly introduced a new piece of behavior into our application, the coverage tools will report that all is well, because the decorator is applied when we define the function, and thus always executed.

In the best case, we’ve tested our rate_limit decorator and thus can say with some confidence that it will work. But if I were to maintain this piece of software, I’d at least like to know whether that decorator has been applied in all the right places.

Coverage-centric testing doesn’t make this necessary. The line is green, and that’s all that counts.


I’m not saying that watching code coverage is bad. It can be a good first indicator on the quality of a contribution. But it isn’t the most important measure of quality, because there are so many ways to cheat at it, and so many ways to do it incorrectly. Manual review is still mostly necessary, and keeping an eye out for coverage-centric testing behaviors is especially valuable to keep the quality of your tests up.


1. There are cases where I deliberately skip writing tests for various reasons, but they come so few and far in between that I’m not going not spend time going into those cases.

2. The following examples require a cursory knowledge of Python, but apply more generally to all kinds of languages.