Hacker News Reader

Comment by throwaway7783 | Hacker News Reader

Why is check if XYZ is called with return value ABC bad, as long as XYZ is an interface method?

Why is a minimally correct fake any better than a mock in this context?

Mocks are not really about order of calls unless you are talking about different return values on different invocations. A fake simply moves the cheese to setting up data correctly, as your tests and logic change.

Not a huge difference either way.

View on HN

The point is to test against a model of the dependency, not just the expected behavour of the code under test. If you just write a mock that exactly corresponds to the test that you're running, you're not testing the interface with the underlying system, you're just running the (probably already perfectly understandable) unit through a rote set of steps, and that's both harder to maintain and less useful than testing against a model of the underlying system.

(And IMO this should only be done for heavyweight or difficult to precisely control components of the system where necessary to improve test runtime or expand the range of testable conditions. Always prefer testing as close to the real system as reasonably practical)

But mocks are a model of the dependency. I don't quite see how a fake is a better model than a mock.

In any case, I agree testing close to a real system, with actual dependencies where possible is better. But that's not done with a fake.

The kind of mocks the OP is arguing against are not really a model of the dependency, they're just a model of a particular execution sequence in the test, because the mock is just following a script. Nothing in it ensures that the sequence is even consistent with any given understanding of how the dependency works, and it will almost certainly need updating when the code under test is refactored.

My point is that a fake doesn't magically fix this issue. Both are narrow models of the underlying interface. I don't still quite understand why a mock is worse than a fake, when it comes to narrow models of the interface. If there is a method that needs to be called with a specific set up, there is no practical difference between a fake and a mock.

Again, none of this is a replacement for writing integration tests where possible. Mocks have a place in the testing realm and they are not an inherently bad tool.

Reply on news.ycombinator.com