Test Driven Development


Recently at work, I’ve been building a complex, spatially-explicit population model. The model is complex enough that I started using Python to program it because R did not easily allow me to program the model. Initially while developing the model, I used informal “testing” to make it produced the correct results. For example, I would write a test script to plot simple results and make sure they outputs looked okay. However, this approach was not satisfactory and it was suggested to me that I use Test Drive Development (TDD).

With TDD, I write a small unit test and then program a function or few of lines of code to answer to test. The test is written in a second script file. After writing the new model code, I run the script test file and make sure the test passes. As part of Python’s “Batteries included” philosophy, base Python even comes with a module for unit testing built in.

This seems simple enough, but I now love TDD! With TDD, I know my code does what I think it is doing (something that is not always easy with complicated models or code). Also, I know (can change my code, but not it’s behavior! For example, if I try to improve a function, I now simply re-run the test script to make sure I didn’t change or break anything.

Although seemingly overkill for simple ecological models, TDD improves the quality and reproducibility of our models. Also, using TDD makes me a better and more confident programmer. My only regret is that I did not start using TDD earlier! For anyone wanting to learn TDD, I found this to be a helpful introduction as well as the Python documentation on unit testing.

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