Reference Testing Exercise 1 (pytest flavour)

Posted on Tue 29 October 2019 in TDDA

This exercise (video 8m 54s) shows how to migrate a test from using pytest directly to the exploiting the referencetest capabilities in the TDDA library. (If you do not currently use pytest for writing tests, you might prefer the unittest-flavoured version of this exercise, since unittest is in Python's standard library.)

We will see how even simple use of referencetest

  • makes it much easier to see how tests have failed when complex outputs are generated
  • helps us to update reference outputs (the expected values) when we have verified that a new behaviour is correct
  • allows us easily to write tests of code whose outputs are not identical from run to run. We do this by specifying exclusions from the comparisons used in assertions.


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

tdda version

to check the version that you have.

Step 1: Copy the exercises

You need to change to some directory in which you're happy to create three new 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-pytest/exercise1  # Go to exercise1

You should have at least the following four files:

$ ls expected.html
  • contains a function called generate_string that, when called, returns HTML text suitable for viewing as a web page.

  • expected.html is the result of calling that function, saved to file

  • contains a single unittest-based test of that file.

  • imports key referencetest functionality from the tdda library into pytest.

It's probably useful to look at the web page expected.html in a browser, either by navigating to it in a file browser and double clicking it, or by using

open expected.html

if your OS supports this. As you can see, it's just some text and an image. The image is an inline SVG vector image, generated along with the text.

Also have a look at the test code. The core part of it is very short:

from generators import generate_string

def testExampleStringGeneration():
    actual = generate_string()
    with open('expected.html') as f:
        expected =
    assert actual == expected

The code

  • calls generate_string() to create the content
  • stores its output in the variable actual
  • reads the expected content into the variable expected
  • asserts that the two strings are the same.

Step 3. Run the test, which should fail

$ pytest      #  This will whether pytest uses Python2 or Python3

You should get a failure, and pytest tries quite hard to show what's causing the failure:

=================================== FAILURES ===================================
_________________________ testExampleStringGeneration __________________________

    def testExampleStringGeneration():
        actual = generate_string()
        with open('expected.html') as f:
            expected =
>       assert actual == expected
E       AssertionError: assert '<!DOCTYPE ht...y>\n</html>\n' == '<!DOCTYPE htm...y>\n</html>\n'
E         Skipping 69 identical leading characters in diff, use -v to show
E         -  Solutions, 2016
E         +  Solutions Limited, 2016
E         ?           ++++++++
E         -     Version 1.0.0
E         ?             ^
E         +     Version 0.0.0...
E         ...Full output truncated (31 lines hidden), use '-vv' to show AssertionError
=========================== 1 failed in 0.11 seconds ===========================

You can certainly see that there's a different in the Version number in the output and also a line including 2016 (a copyright notice, in fact).

But it also says:

...Full output truncated (31 lines hidden), use '-vv' to show

and if you do that, the output becomes a bit overwhelming.

We'll convert the test to use the TDDA libraries referencetest and see how that helps.

Step 4. Change the code to use referencetest.

The key change we need to make is the to the assertion, which will now be:

  • ref.assertStringCorrect(actual, 'expected.html')

ref is object made available by, and is passed into our test function by pytest. We therefore need to change the function declaration to take ref as an argument:

  • def testExampleStringGeneration(ref):

Finally, because assertStringCorrect compares a string in memory to content from a file, we don't need the lines in the middle that read the file:

* Delete the middle two lines of the test function.

The result is:

from generators import generate_string

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

Step 5. Run the modified test

$ pytest

You should see very different output, that includes, near the end, something like this:

E       AssertionError: 2 lines are different, starting at line 5
E       Expected file expected.html
E       Compare raw with:
E           diff /var/folders/zv/3xvhmvpj0216687_pk__2f5h0000gn/T/actual-raw-expected.html expected.html
E       Compare post-processed with:
E           diff /var/folders/zv/3xvhmvpj0216687_pk__2f5h0000gn/T/actual-expected.html /var/folders/zv/3xvhmvpj0216687_pk__2f5h0000gn/T/expected-expected.html

/Users/njr/python/tdda/tdda/referencetest/ AssertionError

(You will probably need to scroll right to see all of the message on this page.)

Because the test failed, the TDDA library has written a copy of the actual ouput to file to make it easy for us to examine it and to use diff commands to see how it actually differs from what we expected. (In fact, it's written out two copies, a "raw" and a "post-precocessed" one, but we haven't used any processing, so they will be the same in our case. So we ignore the second diff command suggested for now.)

It's also given us the precise diff command we need to see the differences between our actual and expected output.

Step 6. Copy the first diff command and run it. You should see something similar to this:

$ diff /var/folders/zv/3xvhmvpj0216687_pk__2f5h0000gn/T/actual-raw-expected.html expected.html
<     Copyright (c) Stochastic Solutions, 2016
<     Version 1.0.0
>     Copyright (c) Stochastic Solutions Limited, 2016
>     Version 0.0.0
< </html>
\ No newline at end of file
> </html>

(If you have a visual diff tool, can also use that. For example, on a Mac, if you have Xcode installed, you should have the opendiff command available.)

The diff makes it clear that there are three differences:

  • The copyright notice has changed slightly
  • The version number has changed
  • The string doesn't have a newline at the end, whereas the file does.

The Copyright and version numbers lines are both in comments in the HTML, so don't affect the rendering at all. You might want to confirm that if you look at the actual file it saved (/var/folders/zv/3xvhmvpj0216687_pk__2f5h0000gn/T/actual-raw-expected.html, the first file in the diff command), you should see that it looks identical.

In this case, therefore, we might now feel that we should simply update expected.html with what generate_string() is now producing. It would be (by design) extremely easy to change the diff in the command it gave is to cp to achieve that.

However, there's better thing we can do in this case.

Step 7. Specify exclusions

Standing back, it seems obvious likely that periodically the version number and Copyright line written to comments in the HTML will change. If the only difference between out expected output and what we actually generate are those, we'd probably prefer the test didn't fail.

The ref.assertStringCorrect function from referencetest gives us several mechanisms for specifying changes that can be ignored when checking whether a string is correct. The simplest one, which will be enough for our example, is just to specify strings which, if they occur on a line in the output, case differences in those lines to be ignored, so that the assertion doesn't fail.

** Step 7a. Add the ignore_substrings parameter to assertStringCorrect as follows:**

        ref.assertStringCorrect(actual, 'expected.html',
                                ignore_substrings=['Copyright', 'Version'])

Step 7b. Run the test again. It should now pass:

$ pytest
============================= test session starts ============================== .                                                            [100%]

=========================== 1 passed in 0.04 seconds ===========================

Recap: What we have seen

We've seen

  1. Converting standard pytest-based tests to use referencetestcase is straightfoward.

  2. When we do that, we gain access to powerful new kinds of assertion such as assertStringCorrect. Among the immediate benefits:

    • When there is failure, this saves the failing output to a temporary file
    • It tells you the exact diff command you need to see be able to see differences
    • This also makes it very easy to copy the new "known good" answer into place if you've verified that the new answer is now correct. (In fact, the library also has a more powerful way to do this, as we'll see in a later exercise).
  3. The ref.assertStringCorrect fucntion also has a number of mechanisms for allowing specific expected differences to occur without causing the test to fail. The simplest of these mechanisms is the ignore_substrings keyword argument we used here.