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Automatically Testing Software


Teaching: 30 min
Exercises: 20 min
  • Does the code we develop work the way it should do?

  • Can we (and others) verify these assertions for themselves?

  • To what extent are we confident of the accuracy of results that appear in publications?

  • Explain the reasons why testing is important

  • Describe the three main types of tests and what each are used for

  • Implement and run unit tests to verify the correct behaviour of program functions


Being able to demonstrate that a process generates the right results is important in any field of research, whether it’s software generating those results or not. So when writing software we need to ask ourselves some key questions:

If we are unable to demonstrate that our software fulfills these criteria, why would anyone use it? Having well-defined tests for our software is useful for this, but manually testing software can prove an expensive process.

Automation can help, and automation where possible is a good thing - it enables us to define a potentially complex process in a repeatable way that is far less prone to error than manual approaches. Once defined, automation can also save us a lot of effort, particularly in the long run. In this episode we’ll look into techniques of automated testing to improve the predictability of a software change, make development more productive, and help us produce code that works as expected and produces desired results.

What Is Software Testing?

For the sake of argument, if each line we write has a 99% chance of being right, then a 70-line program will be wrong more than half the time. We need to do better than that, which means we need to test our software to catch these mistakes.

We can and should extensively test our software manually, and manual testing is well-suited to testing aspects such as graphical user interfaces and reconciling visual outputs against inputs. However, even with a good test plan, manual testing is very time consuming and prone to error. Another style of testing is automated testing, where we write code that tests the functions of our software. Since computers are very good and efficient at automating repetitive tasks, we should take advantage of this wherever possible.

There are three main types of automated tests:

For the purposes of this course, we’ll focus on unit tests. But the principles and practices we’ll talk about can be built on and applied to the other types of tests too.

Set Up a New Feature Branch for Writing Tests

We’re going to look at how to run some existing tests and also write some new ones, so let’s ensure we’re initially on our develop branch we created earlier. And then, we’ll create a new feature branch called test-suite off the develop branch - a common term we use to refer to sets of tests - that we’ll use for our test writing work:

$ git checkout develop
$ git branch test-suite
$ git checkout test-suite

Good practice is to write our tests around the same time we write our code on a feature branch. But since the code already exists, we’re creating a feature branch for just these extra tests. Git branches are designed to be lightweight, and where necessary, transient, and use of branches for even small bits of work is encouraged.

Later on, once we’ve finished writing these tests and are convinced they work properly, we’ll merge our test-suite branch back into develop.

Inflammation Data Analysis

Let’s go back to our patient inflammation software project. Recall that it is based on a clinical trial of inflammation in patients who have been given a new treatment for arthritis. There are a number of datasets in the data directory recording inflammation information in patients (each file representing a different trial), and are each stored in comma-separated values (CSV) format: each row holds information for a single patient, and the columns represent successive days when inflammation was measured in patients.

Let’s take a quick look at the data now from within the Python command line console. Change directory to the repository root (which should be in your home directory ~/python-intermediate-inflammation), ensure you have your virtual environment activated in your command line terminal (particularly if opening a new one), and then start the Python console by invoking the Python interpreter without any parameters, e.g.:

$ cd ~/python-intermediate-inflammation
$ source venv/bin/activate
$ python3

The last command will start the Python console within your shell, which enables us to execute Python commands interactively. Inside the console enter the following:

import numpy as np
data = np.loadtxt(fname='data/inflammation-01.csv', delimiter=',')
(60, 40)

The data in this case is two-dimensional - it has 60 rows (one for each patient) and 40 columns (one for each day). Each cell in the data represents an inflammation reading on a given day for a patient.

Our patient inflammation application has a number of statistical functions held in inflammation/ daily_mean(), daily_max() and daily_min(), for calculating the mean average, the maximum, and the minimum values for a given number of rows in our data. For example, the daily_mean() function looks like this:

def daily_mean(data):
    """Calculate the daily mean of a 2D inflammation data array for each day.

    :param data: A 2D data array with inflammation data (each row contains measurements for a single patient across all days).
    :returns: An array of mean values of measurements for each day.
    return np.mean(data, axis=0)

Here, we use NumPy’s np.mean() function to calculate the mean vertically across the data (denoted by axis=0), which is then returned from the function. So, if data was a NumPy array of three rows like…

[[1, 2],
 [3, 4],
 [5, 6]]

…the function would return a 1D NumPy array of [3, 4] - each value representing the mean of each column (which are, coincidentally, the same values as the second row in the above data array).

To show this working with our patient data, we can use the function like this, passing the first four patient rows to the function in the Python console:

from inflammation.models import daily_mean


Note we use a different form of import here - only importing the daily_mean function from our models instead of everything. This also has the effect that we can refer to the function using only its name, without needing to include the module name too (i.e. inflammation.models.daily_mean()).

The above code will return the mean inflammation for each day column across the first four patients (as a 1D NumPy array of shape (40, 0)):

array([ 0.  ,  0.5 ,  1.5 ,  1.75,  2.5 ,  1.75,  3.75,  3.  ,  5.25,
        6.25,  7.  ,  7.  ,  7.  ,  8.  ,  5.75,  7.75,  8.5 , 11.  ,
        9.75, 10.25, 15.  ,  8.75,  9.75, 10.  ,  8.  , 10.25,  8.  ,
        5.5 ,  8.  ,  6.  ,  5.  ,  4.75,  4.75,  4.  ,  3.25,  4.  ,
        1.75,  2.25,  0.75,  0.75])

The other statistical functions are similar. Note that in real situations functions we write are often likely to be more complicated than these, but simplicity here allows us to reason about what’s happening - and what we need to test - more easily.

Let’s now look into how we can test each of our application’s statistical functions to ensure they are functioning correctly.

Writing Tests to Verify Correct Behaviour

One Way to Do It?

One way to test our functions would be to write a series of checks or tests, each executing a function we want to test with known inputs against known valid results, and throw an error if we encounter a result that is incorrect. So, referring back to our simple daily_mean() example above, we could use [[1, 2], [3, 4], [5, 6]] as an input to that function and check whether the result equals [3, 4]:

import numpy.testing as npt

test_input = np.array([[1, 2], [3, 4], [5, 6]])
test_result = np.array([3, 4])
npt.assert_array_equal(daily_mean(test_input), test_result)

So we use the assert_array_equal() function - part of NumPy’s testing library - to test that our calculated result is the same as our expected result. This function explicitly checks the array’s shape and elements are the same, and throws an AssertionError if they are not. In particular, note that we can’t just use == or other Python equality methods, since these won’t work properly with NumPy arrays in all cases.

We could then add to this with other tests that use and test against other values, and end up with something like:

test_input = np.array([[2, 0], [4, 0]])
test_result = np.array([2, 0])
npt.assert_array_equal(daily_mean(test_input), test_result)

test_input = np.array([[0, 0], [0, 0], [0, 0]])
test_result = np.array([0, 0])
npt.assert_array_equal(daily_mean(test_input), test_result)

test_input = np.array([[1, 2], [3, 4], [5, 6]])
test_result = np.array([3, 4])
npt.assert_array_equal(daily_mean(test_input), test_result)

However, if we were to enter these in this order, we’ll find we get the following after the first test:

Arrays are not equal

Mismatched elements: 1 / 2 (50%)
Max absolute difference: 1.
Max relative difference: 0.5
 x: array([3., 0.])
 y: array([2, 0])

This tells us that one element between our generated and expected arrays doesn’t match, and shows us the different arrays.

We could put these tests in a separate script to automate the running of these tests. But a Python script halts at the first failed assertion, so the second and third tests aren’t run at all. It would be more helpful if we could get data from all of our tests every time they’re run, since the more information we have, the faster we’re likely to be able to track down bugs. It would also be helpful to have some kind of summary report: if our set of tests - known as a test suite - includes thirty or forty tests (as it well might for a complex function or library that’s widely used), we’d like to know how many passed or failed.

Going back to our failed first test, what was the issue? As it turns out, the test itself was incorrect, and should have read:

test_input = np.array([[2, 0], [4, 0]])
test_result = np.array([3, 0])
npt.assert_array_equal(daily_mean(test_input), test_result)

Which highlights an important point: as well as making sure our code is returning correct answers, we also need to ensure the tests themselves are also correct. Otherwise, we may go on to fix our code only to return an incorrect result that appears to be correct. So a good rule is to make tests simple enough to understand so we can reason about both the correctness of our tests as well as our code. Otherwise, our tests hold little value.

Using a Testing Framework

Keeping these things in mind, here’s a different approach that builds on the ideas we’ve seen so far but uses a unit testing framework. In such a framework we define our tests we want to run as functions, and the framework automatically runs each of these functions in turn, summarising the outputs. And unlike our previous approach, it will run every test regardless of any encountered test failures.

Most people don’t enjoy writing tests, so if we want them to actually do it, it must be easy to:

Test results must also be reliable. If a testing tool says that code is working when it’s not, or reports problems when there actually aren’t any, people will lose faith in it and stop using it.

Look at tests/

"""Tests for statistics functions within the Model layer."""

import numpy as np
import numpy.testing as npt

def test_daily_mean_zeros():
    """Test that mean function works for an array of zeros."""
    from inflammation.models import daily_mean

    test_input = np.array([[0, 0],
                           [0, 0],
                           [0, 0]])
    test_result = np.array([0, 0])

    # Need to use NumPy testing functions to compare arrays
    npt.assert_array_equal(daily_mean(test_input), test_result)

def test_daily_mean_integers():
    """Test that mean function works for an array of positive integers."""
    from inflammation.models import daily_mean

    test_input = np.array([[1, 2],
                           [3, 4],
                           [5, 6]])
    test_result = np.array([3, 4])

    # Need to use NumPy testing functions to compare arrays
    npt.assert_array_equal(daily_mean(test_input), test_result)

Here, although we have specified two of our previous manual tests as separate functions, they run the same assertions. Each of these test functions, in a general sense, are called test cases - these are a specification of:

Also, we’re defining each of these things for a test case we can run independently that requires no manual intervention.

Going back to our list of requirements, how easy is it to run these tests? We can do this using a Python package called pytest. Pytest is a testing framework that allows you to write test cases using Python. You can use it to test things like Python functions, database operations, or even things like service APIs - essentially anything that has inputs and expected outputs. We’ll be using Pytest to write unit tests, but what you learn can scale to more complex functional testing for applications or libraries.

What About Unit Testing in Other Languages?

Other unit testing frameworks exist for Python, including Nose2 and Unittest, and the approach to unit testing can be translated to other languages as well, e.g. FRUIT for Fortran, JUnit for Java (the original unit testing framework), Catch for C++, etc.

Installing Pytest

If you have already installed pytest package in your virtual environment, you can skip this step. Otherwise, as we have seen, we have a couple of options for installing external libraries:

  1. via PyCharm (see “Adding an External Library” section in “Integrated Software Development Environments” episode), or
  2. via the command line (see “Installing External Libraries in an Environment With pip section in “Virtual Environments For Software Development” episode).

To do it via the command line - exit the Python console first (either with Ctrl-D or by typing exit()), then do:

$ pip3 install pytest

Whether we do this via PyCharm or the command line, the results are exactly the same: our virtual environment will now have the pytest package installed for use.

Running Tests

Now we can run these tests using pytest:

$ python -m pytest tests/

Here, we use -m to invoke the pytest installed module, and specify the tests/ file to run the tests in that file explicitly.

Why Run Pytest Using python -m and Not pytest ?

Another way to run pytest is via its own command, so we could try to use pytest tests/ on the command line instead, but this would lead to a ModuleNotFoundError: No module named 'inflammation'. This is because using the python -m pytest method adds the current directory to its list of directories to search for modules, whilst using pytest does not - the inflammation subdirectory’s contents are not ‘seen’, hence the ModuleNotFoundError. There are ways to get around this with various methods, but we’ve used python -m for simplicity.

============================================== test session starts =====================================================
platform darwin -- Python 3.9.6, pytest-6.2.5, py-1.11.0, pluggy-1.0.0
rootdir: /Users/alex/python-intermediate-inflammation
plugins: anyio-3.3.4
collected 2 items                               
tests/ ..                                                                                           [100%]

=============================================== 2 passed in 0.79s ======================================================

Pytest looks for functions whose names also start with the letters ‘test_’ and runs each one. Notice the .. after our test script:

So if we have many tests, we essentially get a report indicating which tests succeeded or failed. Going back to our list of requirements, do we think these results are easy to understand?

Exercise: Write Some Unit Tests

We already have a couple of test cases in test/ that test the daily_mean() function. Looking at inflammation/, write at least two new test cases that test the daily_max() and daily_min() functions, adding them to test/ Here are some hints:

  • You could choose to format your functions very similarly to daily_mean(), defining test input and expected result arrays followed by the equality assertion.
  • Try to choose cases that are suitably different, and remember that these functions take a 2D array and return a 1D array with each element the result of analysing each column of the data.

Once added, run all the tests again with python -m pytest tests/, and you should also see your new tests pass.


def test_daily_max():
    """Test that max function works for an array of positive integers."""
    from inflammation.models import daily_max

    test_input = np.array([[4, 2, 5],
                           [1, 6, 2],
                           [4, 1, 9]])
    test_result = np.array([4, 6, 9])

    npt.assert_array_equal(daily_max(test_input), test_result)

def test_daily_min():
    """Test that min function works for an array of positive and negative integers."""
    from inflammation.models import daily_min

    test_input = np.array([[ 4, -2, 5],
                           [ 1, -6, 2],
                           [-4, -1, 9]])
    test_result = np.array([-4, -6, 2])

    npt.assert_array_equal(daily_min(test_input), test_result)

The big advantage is that as our code develops we can update our test cases and commit them back, ensuring that ourselves (and others) always have a set of tests to verify our code at each step of development. This way, when we implement a new feature, we can check a) that the feature works using a test we write for it, and b) that the development of the new feature doesn’t break any existing functionality.

What About Testing for Errors?

There are some cases where seeing an error is actually the correct behaviour, and Python allows us to test for exceptions. Add this test in tests/

import pytest
def test_daily_min_string():
    """Test for TypeError when passing strings"""
    from inflammation.models import daily_min

    with pytest.raises(TypeError):
        error_expected = daily_min([['Hello', 'there'], ['General', 'Kenobi']])

Note that you need to import the pytest library at the top of our file with import pytest so that we can use pytest’s raises() function.

Run all your tests as before.

Since we’ve installed pytest to our environment, we should also regenerate our requirements.txt:

$ pip3 freeze > requirements.txt

Finally, let’s commit our new file, requirements.txt file, and test cases to our test-suite branch, and push this new branch and all its commits to GitHub:

$ git add requirements.txt tests/
$ git commit -m "Add initial test cases for daily_max() and daily_min()"
$ git push -u origin test-suite

Why Should We Test Invalid Input Data?

Testing the behaviour of inputs, both valid and invalid, is a really good idea and is known as data validation. Even if you are developing command line software that cannot be exploited by malicious data entry, testing behaviour against invalid inputs prevents generation of erroneous results that could lead to serious misinterpretation (as well as saving time and compute cycles which may be expensive for longer-running applications). It is generally best not to assume your user’s inputs will always be rational.

Key Points

  • The three main types of automated tests are unit tests, functional tests and regression tests.

  • We can write unit tests to verify that functions generate expected output given a set of specific inputs.

  • It should be easy to add or change tests, understand and run them, and understand their results.

  • We can use a unit testing framework like Pytest to structure and simplify the writing of tests in Python.

  • We should test for expected errors in our code.

  • Testing program behaviour against both valid and invalid inputs is important and is known as data validation.