R Packages, 2nd Edition

Chapter 13. Testing Basics

Testing is a vital part of package development: it ensures that your code does what you want. Testing, however, adds an additional step to your workflow. To make this task easier and more effective this chapter will show you how to do formal automated testing using the testthat package.

The first stage of your testing journey is to become convinced that testing has enough benefits to justify the work. For some of us, this is easy to accept. Others must learn the hard way.

Once you’ve decided to embrace automated testing, it’s time to learn some mechanics and figure out where testing fits into your development workflow.

As you and your R packages evolve, you’ll start to encounter testing situations where it’s fruitful to use techniques that are somewhat specific to testing and differ from what we do below R/.

Why Is Formal Testing Worth the Trouble?

Up until now, your workflow probably looks like this:

1. Write a function.

2. Load it with devtools::load_all(), maybe via Ctrl/Cmd-Shift-L.

3. Experiment with it in the console to see if it works.

4. Rinse and repeat.

While you are testing your code in this workflow, you’re only doing it informally. The problem with this approach is that when you come back to this code in 3 months to add a new feature, you’ve probably forgotten some of the informal tests you ran the first time around. This makes it very easy to break code that used to work.

Many of us embrace automated testing when we realize we’re refixing ...