The reason for having automated tests is to know whether or not code works. Working code fulfills product requirements.
As listener Lindsey, When I tap on the ListenWith app icon, Then I see the names of all my Spotify playlists, So that I know what my listening options are.
Acceptance tests assure me these requirements are met. They automate what Lindsey does when she wants to see her Spotify playlists and then asserts what she expects.
- Tap the ListenWith app icon
- Assert Lindsey’s Spotify playlist names are displayed
When this test passes we know the code works. When it fails, we know the code does not work. It’s indicative of a fulfilled product requirement and that’s reassuring.
Consider these two unit tests instead:
- Tap the ListenWith app icon
- Assert that
func begin()is called
- Assert that Lindsey’s Spotify playlist names are displayed
Or, to put it another way:
Given A causes B And B causes C Then A causes C
Transitive assertions are concessions in comparison with acceptance tests because they’re not indicative of working code. They’re indicative of working code which works in a certain way. When these tests pass, we know that the code works. When these fail, we either know:
- The code does not work
- The code does not work the same way it used to work
If I rename
func begin() to
func start() the code still works yet both tests
fail. If I change
func begin() to call
func completeSetup(), which then
displays Lindsey’s playlists, the latter unit test fails, yet the code still
When the product requirements remain the same, the code continues working, yet the test suite begins failing, you’re wasting your time figuring out why 50% of the time - and agile is about reducing waste. The code may actually no longer work or it may just not work the same way.
There are pragmatic reasons for unit tests.
Adhering strictly to TDD requires unit testing and teaches you how to write simpler code which junior programmers love to avoid.
Sometimes acceptance tests take an unacceptably long time to run. This is easily the most commonly used argument against acceptance tests.
I’d rather have a test suite which takes 20 minutes to run and tells me with certainty whether or not my code works than a suite which takes 10 minutes to run and tells me that maybe my code works.
Sure, when your 10 minute suite passes, you absolutely just saved 10 minutes.
However, any tests failure will probably cost you more than 10 minutes to figure out if the code actually works or the tests are stale. I’d rather know in 20 minutes for sure. I’ll spend those 20 minutes making tangible progress learning about another subject by reading a blog post.
Do the best thing for today.
If your suite is 10 seconds long and the next product requirement’s acceptance test will add 5 seconds, while the unit testing method will add 0.3 seconds, choose the acceptance test. It’s still saving you time at this point. There may come a time to look back. Think deeply about when to do that. It’s not today.