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unittest, part 3

·3 mins

So far, we’ve talked about TestSuites, TestCases and TestResults. We’ve seen how these objects interact with each other  and how they can generally be thought about as having more than one interface. TestResult has an interface for the TestCase and an interface used for querying the results, TestCase has an interface for test runners and an interface for test authors, and so forth.

Now we need to give some time to the bits that glue everything together: the test runner and the test loader.


You will not find a class in called TestRunner. A test runner is simply something that takes user input about a test run – what tests to run, what manner to run them in, how to display the results – and does it.

Essentially, it does something like this:

  test = TestLoader().loadTests(user_specified_test_string)  result = makeTestResult(options_specified_by_user)  result.startTestRun()  try:  finally:    result.stopTestRun()

And that’s it.

You see that the test runner is responsible for instantiating the test loader and the test result. It’s perhaps excusable for a test runner to be tightly bound to particular implementations of test loader and test result. Certainly, before TestResult grew startTestRun and stopTestRun it was inevitable: since the test runner was responsible for summarizing the results of a test run, overall responsibility for displaying the results was split between the runner and the result.

Nowadays, the tight coupling can be limited. If your test runner has an option to display stack traces as it gets them, then that’s pretty much going to force you to use a particular result. However, you can still write your code internally such that someone could pass in a different result that still works, even though it doesn’t do exactly what the user asked for.


From the point of view of interfaces and compatibility, this is a pretty boring class, and that’s a good thing. The test loader’s job is to find tests based on some user input and construct a single ITest object for them.

When it does more than this, one runs the risk of having the behaviour of a test suite depend too much on the runner itself. The ideal is to have the test suite run in any runner: trial, nose, unittest2, py.test, whatever.

Some TestLoaders provide hooks so that users with complicated test suites can customize the way their tests are loaded. Whenever the Trial TestLoader sees a test_suite() function in a module, it lets that function take charge of the loading.

The standard library in 2.7 has a new hook, inspired by an innovation in bzrlib, but slightly different. load_tests(loader, standard_tests, pattern) is given the loader used by the test runner, the tests that the loader would have loaded, and if appropriate, a glob used for matching test module files. The advantage of this hook is that it reduces the danger of customizations made to the loader, since the test suite has access to the same loader. It also makes custom loading easier by giving the standard tests as a starting point. bzrlib uses this to run the same set of tests against many implementations.

I think that’s all I have to say about these two, which means that’s pretty much all I have to say about unittest’s API for test frameworks. Still one more post to go though: interfaces for test authors.

Let me know if I’ve missed anything, if anything here surprises you or contradicts something I said in the past or if things are unclear. The comments on the previous two posts have really helped!