Single expectation test

[andreareginato]

Write your thoughts about the "single expectation test" best practice.

[sj26]

This can, however, be the bane of performant testing.

[josephlord]

I'm fairly new and inexperienced with Rspec and especially where there is some significant set up it can make it very slow. I have sometimes written as separate tests during TDD, got it working and then written a combined test to leave running in CI process/automatic testing and commented out the separate tests. If the combined test fails I can quickly activate the separate tests to find the exact issue.

[davearonson]

I sometimes use an assertion to verify that the conditions are right for the test to be valid. For instance, on one project, we had a very complicated set of fixtures, in several related models. There were about a dozen devs, so we were often stepping on each other's fixtures. When I tested a scope, I would usually do "it finds only those that fit" and "it omits only those that don't fit", by looping through the found or omitted records, and verifying that they fit or not. However, this tactic depends on there being ANY records that do or don't fit. So, I preceded the loop with a test that the set contained at least one record. One of our pull-request-approvers regularly made the perfect the enemy of the good, strictly enforcing "one assert per test", and rejected that style, so I made the existence-check a separate test, much slower. (To those who say putting the assertion in a loop was already a violation, yes I could have set a flag upon error and checked it at the end of the loop, but that cuts out a lot of clue about which record violated it. Yes, I could have initted something to nil and set it to the offending record's id on error, but still....)

[myronmarston]

I think this best practice, as written, is misleading and not helpful. Here's what I'd say instead:

• In isolated unit specs, you want each example to specify one (and only one) behavior. Multiple expectations in the same example are a signal that you may be specifying multiple behaviors.
• In tests that are not isolated (e.g. ones that integrate with a DB, an external webservice, or end-to-end-tests), you take a massive performance hit to do the same setup over and over again, just to set a different expectation in each test. In these sorts of slower tests, I think it's fine to specify more than one isolated behavior.

As in all things, software engineering is about tradeoffs, and this is no exception. It's bad to encourage cargo-cult thinking here.

[brycemcd]

+1 to @myronmarston

[thermistor]

+1 to @myronmarston

[coreyhaines]

One of the primary reasons for the "single assertion per test" is to provide easy and understandable traceability for what caused the failure, and keep multiple errors from mingling together and confusing this feedback.
While I do agree that single assertion tests are generally a good thing, I warn people to stay away from it as a guideline and instead focus on the goal: making test failures point directly to the cause of the error.

[jandudulski]

👍 to @myronmarston

[mike-burns]

Later you make the point that you should have custom matchers. That is actually a very pragmatic suggestion that accomplishes a similar goal to this suggestion.

[pelle]

+1 to @myronmarston

[larrylv]

+1 to @myronmarston

[rilian]

+1 to @myronmarston

[andreareginato]

Updated using the @myronmarston description.
Any correction and comment to the updated guideline is appreciated.