Question

(I’m using rspec in RoR, but I believe this question is relevant to any testing system.)

I often find myself doing this kind of thing in my test suite:

actual_value = object_being_tested.tested_method(args)
expected_value = compute_expected_value(args)
assert(actual_value == expected_value, ...)

The problem is that my implementation of compute_expected_value() often ends up mimicking object_being_tested.tested_method(), so it’s really not a good test because they may have identical bugs.

This is a rather open-ended question, but what techniques do people use to avoid this trap? (Points awarded for pointers to good treatises on the topic…)

Answer 1

Usually (for manually written unit tests) you would not compute the expected value. Rather, you would just assert against what you expect to be the result from the tested method for the given args. That is, you would have something like this:

actual_value = object_being_tested.tested_method(args)
expected_value = what_you_expect_to_be_the_result
assert(actual_value == expected_value, ...)

In other testing scenarios where the arguments (or even test methods being executed) are generated automatically, you need to devise a simple oracle which will give you the expected result (or an invariant that should hold for the expected result). Tools like Pex, Randoop, ASTGen, and UDITA enable such testing.