I want to stop people abusing Python's doctest format. Many of the tests I've seen written as doctest files would have been better off as plain unittest files. I'm going to try explain why. I have many gripes about how people use doctests, but probably the biggest is that narrative tests are lousy unit tests.
Narratives tell a story. Something happens, then another thing, and another thing, one after the other, in sequence. Earlier events influence later ones as the story gradually assembles a complete picture. Humans like stories, our brains are used to telling them and receiving them.
Technical documentation is often written with a narrative. Tutorials are an obvious case, but not the only one. A guide to an API may show a series of different examples, each contrasting with the others in ways that explain to the reader what they need to understand.
Automated tests can have narratives too, of course. A narrative test is quite easy to write: write some code that does something (and check the result), then do something else (and check that result), and so on until you've done (and checked) everything you want to do (and check). Doctests make this particularly easy. Here's a toy example of a doctest:
Instantiate a Frobber. >>> frobber = Frobber() >>> frobber.has_frobbed() False Now frob it. >>> frobber.frob() >>> frobber.has_frobbed() True It can't be frobbed twice. >>> frobber.frob() Traceback (most recent call last): ... AlreadyFrobbedError: ...
Narrative tests can be good acceptance tests. An acceptance test often takes the form of a story; an example might be “an unlogged in user visits a web page. They click a particular link that needs a logged in user, so they get taken to a login screen. The user has no account yet, so they walk through the account creation wizard. Once the wizard is completed, the account is created and they logged in, and they are taken to the link they originally clicked on.”
So, having shown how they are easy to write, and appropriate for some tests, I'll now explain why narratives make lousy unit tests.
A typical unit test has four phases:
- Set up a fixture
- Interact with the system-under-test
- Verify the outcome
- Tear down the fixture
Good unit tests are small and specific: they will test just one condition per test method, i.e. the X and the Y will be as minimal as reasonably possible. There's considerable benefit to this style:
- Every individual test has a name. I can refer to a failing test precisely by name when communicating with my fellow developers. I can communicate the name to the test runner too: when I am trying to focus on just one problem, it's extremely useful to be able to easily and precisely specify the subset of the full suite I want to run, down to just one test if necessary. I can even jump straight to a test method definition with ctags. Compare that with doctests, where you have to say things like “about line 300 of foo-bar.txt” or “Just after where it says ...”. That's awkward and imprecise, especially when developers are often looking at slightly different versions of the same file.
- Specific tests give clearer failures, and are easier to debug. Good unit tests keep the context of the test fairly minimal (Meszaros' xUnit Test Patterns book explicitly describes “General Fixture” as a cause of the “Obscure Test”). Narratives inherently accumulate context with every line, whether it's relevant to anything else or not. You have to be aware of everything that happened earlier in the story to understand and debug a failure (and if this isn't the case, then what's the point in having a narrative?). Unit tests also tend to generate more relevant failures, because only tests that are actually affected by the problem fail, rather than everything after line 100 because that's where the first failure was (and if you suppress the secondary failures, you may be suppressing interesting ones along with the irrelevant ones).
- Specific, narrow tests are better at communicating intent and ensuring converage. If each test is there to verify just one condition, then you can't accidentally lose test coverage just by “tidying” the code (automated coverage analysis tools won't necessarily notice either; there's more to coverage than just tracking lines executed). If you have long, rambling tests, there's a tendency to have a bunch of stuff that's exercised only implicitly, as a side-effect of doing it all in one big eager narrative... so changes to that narrative can easily lose that coverage. Simple, specific code is easier to maintain than single a meandering story that tries to hit as many cases as possible. Make single-condition unit tests an explicit part of your coding standard!
So that's why I think narrative tests are poor unit tests. And I think unit tests ought to be the bulk of most automated test suites.
Tomorrow I'll post about some other problems with the doctest format.
Last modified: 22 October 2008
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