[sirsinsalot]

Unit tests assert implementation behaviour to aid refactoring. If developers misunderstand the spec, the unit tests can be valid. They don't assert developer understanding.

Say it with me, unit tests are to aid refactoring.

If we mix QA and implementation details just because both sides use the word "test" it ends in trouble.

QA should be blind to unit test coverage or even usage at all, they're totally independent concerns.

A passing unit test says nothing against correctness of product against a spec or design... only that it works and continues to work as a developer intended, to aid the work of future developers, even if they misunderstood the spec.

Your comment is at the core of why QA is a total mess. Everyone is confused about what "test" means in different contexts.

Why have a QA function at all with 100% unit test coverage? Because the unit tests may encode misunderstanding by developers. They're there to fight entropy, not wrongness.

QA, using BDD and other tools, ensure the product is correct, regardless of how well it fights entropy by unit tests.

[BlackFly]

Unit tests (class or method as the unit) hinder refactoring by binding to low level implementation details. When you refactor, by definition you are changing what the factors (units) are. Generally, your unit tests will then be testing implementation details that no longer exist. By strongly coypling to implementation details, unit test suites suffer an extremely large ripple effect on refactoring.

Tests in general can only help you refactor code at a lower level of granularity than what you are testing. Something lower than unit level is a contradiction.

Of course, you can instead test business behavior which isn't as volatile in refactoring and change your definition of unit to be a unit of practical business requirements...

[sirsinsalot]

If a unit test covers function f(a,b) to ensure it always returns the right answer in the domain of a,b

But a developer looks at f(a,b) and realises a new implementation could be 10x faster.

The developer re-writes the function, the tests still pass. Without that test they couldn't be sure their rewrite didn't break the expected behaviour.

What you're talking about is changing interfaces and structure when refactoring. Yes, unit tests can make that more painful. But you bin your old tests and write new ones.

If your unit tests are not cheap to dispose of, run or rewrite ... that's your core problem.

If your unit tests are not testing discrete units, instead testing the combined behaviour of many units (the re-factoring of structure pain) ... then you have bigger issues than the tests; namely understanding the difference and applicability between unit and integration tests.

[hbrn]

> If your unit tests are not testing discrete units, instead testing the combined behaviour of many units

This is a very naive idea of unit tests. In the real world applications have a dependency hierarchy with more than two levels. If you want to test anything other than the leaves in that hierarchy you are by definition testing the behaviour of many units.

Sounds like what you're saying is that unit tests are only applicable to leaves in the dependency hierarchy. I could agree with that, but that's not what the world describes as unit tests. That would also make unit tests quite useless.

[sirsinsalot]

> Thay would also make unit tests quite useless

Not so. Their use is to aid refactoring of those nodes. Also the leaf node can contain calls to other code (leafish) if that code too is unit tested.

They're the sum of their parts.

Integration tests used lightly to test specific integration occurs (ie f() not only meets X constraint but calls y() in a certain way to do so) if that's important.

The fact that most codebases unit tests are a fragile mess, and that many devs find tests a burden highlights just how poorly the topic is understood.

[hbrn]

> Their use is to aid refactoring of those nodes

Sure. But there are codebases where leaves are less than 20% of the code. Yet testing preachers are saying that unit tests are the most important part (e.g. Martin Fowler's Testing Pyramid). Though again, they use a different definition of unit tests: everything is a unit if you mock enough.

> how poorly the topic is understood

Well, at least one of the reasons is that people focus too much on test classification. You need to decide what is important to test, how is just implementation detail. If that's your approach, you'll realize that you're getting the most value from integration tests. The only downside to this is losing Testing Church membership. But that happens when you apply logic and reason.

Instead, everybody decides that "we write unit tests", without thinking what needs to be tested and recognizing limitations of unit tests.

[ramesh31]

>Your comment is at the core of why QA is a total mess. Everyone is confused about what "test" means in different contexts.

Particularly the important differences between unit, integration, and e2e testing. Many people use the words interchangeably when they are completely different concerns.

Unit test your library code that has no external dependencies, integration test your classes that implement those libraries, and e2e test your application that is built with those classes. There are varying philosophies to which are more useful, but it's an important distinction to maintain in terminology.