There’s nothing like working on testing to get you familiar with a codebase. I’ve been working on adding back in some testing to the ZenML codebase this past couple of weeks and as a relatively new employee here, it has been a really useful way to dive into how things work under the hood.

This being my first time working seriously with Python, there were a few things that I had to learn along the way. What follows is an initial set of lessons I took away from the experience.

1. One-size-fits-all won’t cut it

Looking at things from a higher level, it’s important to realise that there are lots of different approaches that you could take to testing. It’s a truism that you should ‘test intent, not implementation’, but I imagine that in some scenarios like for software being deployed on a space shuttle you’d want to maybe also test the implementation as well.

Similarly, different companies and projects have different needs for testing. If you’re a huge company, testing is a way of ensuring reliability and preventing catastrophic failures along the way. If you’re a small company, where speed of creation and the pace of development is frantic, having too rigid a set of tests may actually end up hurting you by stifling your ability to iterate through ideas and changes quickly.

I found it helped to take a step back early on in my testing to really think through what I was doing, why I was doing it, and what larger goal it was there to support.

2. ‘Don’t be that person’: testing to crush the spirits of your team

It’s worth reiterating the previous remark about testing intent and not implementation.

If you test every last conditional statement, checking that the code is built in exactly that specific way, changing anything in the original codebase is going to become incredibly tiresome. Moreover, your testing library will start to resemble a kind of byzantine twin replica of your original code.

For preventing this, it helps if everyone in the team is testing as much as they are writing new code. This way it is just part of the development process and not a separate add-on from a QA-like team. At ZenML, we’re small enough that the expectation is that if you work on a new feature, you should also be responsible for writing the tests that go alongside.

3. Pytest, O Pytest!

Pytest is amazing. It has everything you need to write your tests, is easy to understand, and has great documentation of even the slightly more niche features. Can you tell I really enjoyed getting to know this open-source library?

For now, I’ll mention some of the really useful combinations of CLI commands that I found useful.

# make the test output verbose
pytest tests/ -v

# stop testing whenever you get to a test that fails
pytest tests/ -x

# run only a single test
pytest tests/test_base.py::test_initialization

# run only tests tagged with a particular word
pytest tests/ -m specialword

# print out all the output of tests to the console
pytest tests/ -s

# run all the tests, but run the last failures first
pytest tests/ --ff

# see which tests will be run with the given options and config
pytest tests/ —collect-only

# show local variables in tracebacks
pytest tests/ —showlocals

And there are so many more! The flexibility of the CLI tool allows you to be really nimble and ensures you don’t have to hang around for already-passing tests to run.

4. Temp Files & Temp Directory Choice Paralysis

At a certain point I needed to test that certain functions were having side effects out in the real world of a filesystem. I didn’t want to pollute my hard drive or that of whatever random CI server was running the tests, so then I started looking around for options for the creation of temporary files and directories.

It turns out that between the Python standard library, Pytest and some library-specific features, we’re spoiled for choice when it comes for convenience helpers to create temporary files and directories. Python has tempfile which is a platform-agnostic way of creating temporary files and directories. Pytest has tmp_path which you can insert as an argument into your test function and have a convenience location which you can use to your heart’s content. (There are also several other options with Pytest). Then other libraries you’re using may have specific testing capabilities. We use click for our CLI functionality and there’s a useful convenience pattern for running commands from a temporary directory:

def test_something():
   runner = CliRunner()
   with runner.isolated_filesystem():
      # do something here in your new temporary directory

5. Decorate your way to clearer test code

Pytest has a bunch of helper functions which enhance the test code you already have. For instance, if you want to wanted to iterate over a series of values and pass them in as arguments to a function, you can just use the parametrize functionality:

@pytest.mark.parametrize("test_input,expected", [("3+5", 8), ("2+4", 6), ("6*9", 42)])
def test_eval(test_input, expected):
    assert eval(test_input) == expected

Note that this would fail because 6x9 does not equal to 42.

If you have a test that you know is failing right now, but you want to put it to the side for the moment, you can mark it down as being expected to fail with xfail:

@pytest.mark.xfail()
def test_something() -> None:
	# whatever code you have here doesn't work

I find it’s more useful in this way to get a full sense of which tests aren’t working rather than just commenting them out.

The mark method in general is a great way of creating some custom ways to run your tests. You could — using a @pytest.mark.no_async_call_required decorator — distinguish between tests that take a bit longer to run and tests that are more or less instantaneous, for example.

6. Use hypothesis for random arguments

Hypothesis is a Python library to check that functions work the way you think they do. It works by setting up certain conditions under which the function should work.

For example, you can say that this function should be able to accept any datetime value without any problem. Instead of trying to come up with a list of different possible edge cases, hypothesis instead will run (in parallel) a whole series of values to check that this is actually the case. As the docs state:

“It works by generating arbitrary data matching your specification and checking that your guarantee still holds in that case. If it finds an example where it doesn’t, it takes that example and cuts it down to size, simplifying it until it finds a much smaller example that still causes the problem. It then saves that example for later, so that once it has found a problem with your code it will not forget it in the future.” (source)

These custom ways of testing certain kinds of inputs are called ‘strategies’, and it has a whole bunch of these to choose from. The ones I most often use are text, integers, decimals and datetime.

7. Use tox to test multiple versions of Python

tox allows you to automate running your test suite through multiple versions of Python. It’s likely that your CI process does this as well, so in order to test that these are passing locally as well, you can use tox. It creates new virtual environments using the versions you specify and runs your test suite through each of them.

Note that if you’re using pyenv as your overall Python version manager, you may have to use something like the following command to make sure that all the various Python versions are available to tox:

pyenv local zenml-dev-3.8.6 3.6.9 3.7.11 3.8.11 3.9.6

The first argument passed in is my development environment in which I usually work, but the other Python versions / environments are to make those versions available to tox.

8. Debug your failing tests with pdb

Pytest has a bunch of handy ways of inspecting exactly what’s going on at the point where a test fails. I showed some of those above, where you can show, for example, whatever local variables were initialized alongside the stacktrace.

Another really useful feature is the --pdb flag which you can pass in along with your CLI command. This will deposit you inside a pdb debugging environment at exactly the moment your test fails. Super useful that we get all this convenience functionality out of the box with Pytest.

9. Linting: before and beyond testing

At ZenML we use pre-commit hooks that kick into action whenever you try to commit code. (Check out our pyproject.toml configuration and our scripts/ directory to see how we handle this!) It ensures a level of consistency throughout our codebase, ensuring that all our functions have docstrings, for example, or implementing a standard order for import statements.

Some of this — the mypy hook, for example — starts to verge into what feels like testing territory. By ensuring that functions all have type annotations you sometimes are doing more than just enforcing a particular coding style. When you add mypy into your development workflow, you get up close and personal with exactly how different types are passed around in your codebase.

10. …and remember, coverage is just a number!

It’s always good to have a number to chase. It gives you something to work towards and a feeling of progress. Tools like Codecov offer fancy visualizations of just which parts of your codebase still need some attention. Automating all this as part of the CI process can highlight when you’ve just added a series of features but no accompanying tests.

Bearing all these positives in mind, you should still always remember that your tests are there to serve your broader goals. If your goal is to rapidly iterate and create new features, maybe having a goal of 100% test coverage at all times is an unrealistic expectation. A 100% test coverage does not necessarily mean your code is bug-free and robust. It just means that you invoked it during the testing process.

Similarly, different kinds of codebase will have different kinds of test weightings. We didn’t really talk much about the different types of tests (from unit to integration to usability), but some systems or types of designs will require more focus on different pieces of this bigger picture.

Alex Strick van Linschoten is a Machine Learning Engineer at ZenML.


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