Unit Tests Considered Harmful

As some have noted, I’m not actually against tests. Quite the contrary - I’ve been practicing TDD since 2013. My approach to software is driving emergent design by TDD, initially via an E2E test, then via fast, integrative acceptance tests that exercise features while faking IO. I do write unit tests for very specific and pure pieces of software that embody a business domain, such as algorithms or validations.

What I abhor is compulsively writing mockist unit tests for all classes or functions with the goal of satisfying some coverage metrics. These are hurting you thrice: you waste time writing them and maintaining them; you get a false sense of confidence because you have “full coverage”; and when trying to refactor, they hold you back.

You can read more about my approach here: https://www.shaiyallin.com/post/fake-don-t-mock

So what? Every implementation will have some basic structure. That structure can be modified if needed. The point of a unit test is not to posit that any particular thing exists, but that the things that do exist actually work.

>In my experience, most people write unit tests for internal implementation details that are irrelevant to the business domain. They end up inhibiting code change rather than encouraging change, because anything changes often require re-evaluating each failing test to see if it was meaningful in the first place - and if 100s of tests are no longer meaningful, it's easy to skip the couple of tests which are true regression tests.

If the internal implementation is not observable without a bunch of other bullshit, these tests can help instill confidence that the stuff actually works. If you don't test, or test the overall system, it takes much longer. There is such a thing as a pointless test but it is far more common in my experience to have stuff that isn't covered at all by tests. If your biggest problem is that you have to delete some tests that you made obsolete, that's perfectly ok.

>As the author writes, full end-to-end tests "are often slow, cumbersome, hard to debug and tend to be flaky."

Those types of tests are not unit tests.

>Instead, find the internal interfaces which tied to the business logic ("something that delivers value to a paying client"), and write the tests to that. You can use unit test frameworks for that sort of functional testing.

It isn't only the business logic that needs to be tested. Anything that is cumbersome to test "enough" in the overall system ought to be unit tested. At work I'm faced with a series of structures that are cumbersome to test in isolation and in totality. If I had unit tests I could make changes at least 3x faster.

Setting aside functional vs OO paradigms, even if you have a simple helper function like:

  def removeprefix(s, prefix):
    if s.startswith(prefix):
      return s[len(prefix):]
    return s

If you write tests for the function then you hinder future refactoring from removeprefix(s, prefix) to s.removeprefix(prefix) once you switch to a Python which implements s.removeprefix (3.9, I think?)

For example, in red-green-refactor TDD, you are not supposed to change the tests when you refactor.

What you've ended up with are tests for the specific structure, and not the general goals.

> If you don't test, or test the overall system, it takes much longer.

Good thing neither I nor the linked-to author makes either of those arguments.

> it is far more common in my experience to have stuff that isn't covered at all by tests

Which is why I use coverage-based method to identify what need more tests. Coverage is a powerful tool, and easily misused.

> If your biggest problem is that you have to delete some tests that you made obsolete, that's perfectly ok.

The biggest problem is that you decide to not refactor because there are so many tests to delete, and you have to figure out if the failing test really is okay to delete (because it was implementation specific) vs. one that needs to be updated (eg, because it handles a regression case).

> Those types of tests are not unit tests

Correct! And no one said they were.

> At work I'm faced with a ...

That's fine. You do you.