R Packages, 2nd Edition

Chapter 15. Advanced Testing Techniques

This chapter runs a few small bits of testthat code, so we must do some setup that is not necessary in organic testthat usage.

Warning

Your test files should not include these library() calls. We also explicitly request testthat edition 3, but in a real package this will be declared in DESCRIPTION:

library(testthat)
local_edition(3)

Test Fixtures

When it’s not practical to make your test entirely self-sufficient, prefer making the necessary object, logic, or conditions available in a structured, explicit way. There’s a preexisting term for this in software engineering: a test fixture.

A test fixture is something used to consistently test some item, device, or piece of software. — Wikipedia

The main idea is that we need to make it as easy and obvious as possible to arrange the world into a state that is conducive to testing. We describe several specific solutions to this problem:

• Put repeated code in a constructor-type helper function. Memoise it, if construction is demonstrably slow.

• If the repeated code has side effects, write a custom local_*() function to do what’s needed and clean up afterwards.

• If the preceding approaches are too slow or awkward and the thing you need is fairly stable, save it as a static file and load it.

Create useful_things with a Helper Function

Is it fiddly to create a useful_thing? Does it take several lines of code but not much time or memory? In that case, write a helper function to create a useful_thing ...