Reference Testing Exercise 2 (unittest flavour)

Posted on Wed 30 October 2019 in TDDA

This exercise (video 3m 34s) shows a powerful way to run only a single test, or some subset of tests, by using the @tag decorator available in the TDDA library. This is useful for speeding up the test cycle and allowing you to focus on a single test, or a few tests. We will also see, in the next exercise, how it can be used to update test results more easily and safely when expected behaviour changes.

(If you use pytest for writing tests, you might prefer the pytest-flavoured version of this exercise.)


★ You need to have the TDDA Python library (version 1.0.31 or newer) installed see installation. Use

tdda version

to check the version that you have.

Step 1: Copy the exercises (if you don't already have them)

You need to change to some directory in which you're happy to create three directories with data. We are use ~/tmp for this. Then copy the example code.

$ cd ~/tmp
$ tdda examples    # copy the example code

Step 2: Go the exercise files and examine them:

$ cd referencetest_examples/exercises-unittest/exercise2  # Go to exercise2

As in the first exercise, you should have at least the following three files

$ ls
  • expected.html contains the expected output from one test,
  • contains the code to be tested,
  • contains the tests.

If you look at, you'll see it contains two test classes with five tests between them. Only one of the tests is useful (testExampleStringGeneration) with all the others making manifestly true assertions and deliberately wasting time to simulate annoyingly slow tests.

import time
from tdda.referencetest import ReferenceTestCase, tag
from generators import generate_string

class TestQuickThings(ReferenceTestCase):

    def testExampleStringGeneration(self):
        actual = generate_string()
        self.assertStringCorrect(actual, 'expected.html')

    def testZero(self):

class TestSuperSlowThings(ReferenceTestCase):

    def testOne(self):
        self.assertEqual(1, 1)

    def testTwo(self):
        self.assertEqual(2, 2)

    def testThree(self):
        self.assertEqual(3, 3)

Step 3: Run the tests, which should be slow and produce one failure

$ python   #  This will work with Python 3 or Python2

When you run the tests, you should get a single failure, that being the non-trivial test testExampleStringGeneration from the class TestQuickThings.

The output will be:


[...details of test failure...]

Ran 5 tests in 6.007s
FAILED (failures=1)

We get a test failure because we haven't added the ignore_substrings parameter that we saw in Exercise 1 is needed for it to pass.

The tests should take slightly over 6 seconds in total to run, because of the annoyingly slow tests in TestSuperSlowThings. (If you're not annoyed by a 6-second delay, increase the sleep time in one of the "slow" tests until you are annoyed!)

The point of this exercise is to show some simple but very useful functionality for running only tests on which we wish to focus, such as our failing test.

Step 4: Tag the failing test using @tag

If you look at the import statements, you'll see that as well as ReferenceTestCase we also import tag. This is a decorator function1 that we can put before individual tests, or test classes, to indicate that they are of special interest temporarily.

Edit to decorate the failing test by adding @tag on the line before it, thus:

class TestQuickThings(ReferenceTestCase):

    def testExampleStringGeneration(self):
        actual = generate_string()
        self.assertStringCorrect(actual, 'expected.html')

    def testZero(self):

Step 5: Run only the tagged test

Having tagged the failing test, if we run the tests again adding -1 (the digit one, for "single",not the letter ell) to the command, it will run only the tagged test, and take hardly any time. The (abbreviated) output should be something like

$ python -1

[...details of test failure...]

Ran 1 tests in 0.006s
FAILED (failures=1)

You can also use --tagged instead of -1 if you like more descriptive flags.

We can tag as many tests as we like, across any number of test files, and we can also tag whole classes by placing the @tag decorator before a test class definition. So if we instead use:

class TestQuickThings(ReferenceTestCase):

    def testExampleStringGeneration(self):
        actual = generate_string()
        self.assertStringCorrect(actual, 'expected.html')

    def testZero(self):

and run the tests with -1, we will get output more like:

$ python -1

[...details of test failure...]

Ran 2 tests in 0.006s
FAILED (failures=1)

In this case, both the tests in our first test class were run, but no others (and, in particular, not our painfully slow tests!)

Step 6: Locating @tag decorators

In a typical debugging or test development cycle in which you have been using the @tag decorator to focus on just a few failing tests, you might end up with @tag decorations scattered across several files, perhaps in multiple directories. (We're assuming here you have or similar that imports all the other test classes so you can easily run them all together.)

Although it's not hard to use grep or grep -r to find them, the library can actually do this for you. If you use the -0 flag (the digit zero, for "no tests"), or the --istagged flag, instead of running the tests, the library will report which test classes in which files have tagged tests. So in our case:

$ python -0



Here, __main__ stands for the current file; other files would be referenced by their imported name.

Recap: What we have seen

This simple exercise has shown how we can easily run subsets of tests by tagging them and then using the -1 flag (or --tagged) to run only tagged tests.

In this case, the motivation was simply to save time and reduce clutter in the output, focusing on one test, or a small number of tests.

In the Exercise 3, we will see how this combines with the ability to automatically regenerate updated reference outputs to make for a safe and efficient way to update tests after code changes.

  1. Decorator functions in Python are functions that are used to transform other functions: they take a function as an argument and return a new function that modifies the original in some way. Out decorator function tag is called by writing @tag on the line before function (or class) definition, and the effect of this is that the function returned by @tag replaces the function (or class) it precedes. In our case, all @tag does is set an attribute on the function in question so that the TDDA reference test framework can identify it as a tagged function, and choose to run only tagged tests when so requested.