Writing your first unit test

Last updated on 2025-12-05 | Edit this page

Overview

Questions

  • What does a unit test look like?

Objectives

  • Understand the benefits of parameterized tests.
  • Able to write a unit test which is isolated, minimal and fast.

The key aspects of a unit test are the same no matter the language being testing (python, Fortran, etc) or the framework we are using (pFUnit, etc). Therefore, when we are first learning unit testing, it can be useful to think about what the content of a unit test might look like before we try to learn the specific syntax of any one tool.

Testing the temperature

We’ll now use an example Fortran library which converts between units of temperature. This code can be found in the exercises repo under 3-writing-your-first-unit-test/challenge/src/temp_conversions.f90. This library contains two functions, one to convert from Fahrenheit to Celsius (fahrenheit_to_celsius) and another to convert from Celsius to Kelvin (celsius_to_kelvin).

Imagine we want to use this library to do some temperature conversions from Fahrenheit to Kelvin. To ensure the library contains the functionality we need, we decide to write some unit tests.

Challenge

Challenge: Pseudo test

Write a unit test in pseudocode for the temperature library to check that it can convert from Fahrenheit to Kelvin.

Your test could look something like this…

Set some input value of Fahrenheit, for example 32.0
Call fahrenheit_to_celsius with this input
Check that the output is equal to the expected value of 0.0

Set some input value of Celsius, for example 0.0
Call celsius_to_kelvin with this input
Check that the output is equal to the expected value of 273.15

Writing a test

All unit tests tend to follow a similar pattern.

  1. Define the inputs to your unit of code to be tested as well as the outputs you expect from execution with these inputs.

  2. Setup and verify any state required for successful execution (verify a file exists, allocate memory, etc)

  3. Call the unit of code to be tested using the inputs defined in the first step.

  4. Verify the actual outputs of your unit of code with the expected outputs defined in the first step.

Challenge

Challenge: Standard Fortran test

Write a unit test in standard Fortran for the temperature library to check that it can convert from Fahrenheit to Kelvin. You can use your pseudocode as a starting point.

As we are not yet using a testing framework, some boilerplate code has been provided to help you create a test-suite. Take a look at part one of the exercise 3-writing-your-first-unit-test/challenge.

A solution is provided in 3-writing-your-first-unit-test/solution.