R testthat: use external package only in test file - not in DESCRIPTION

Question

I'm trying to run a testthat script using GitHub Actions.

I would like to test a functionality of my function that allows it to be combined with (many) external packages. Now I want to test these external packages for the R CMD Check but I don't want to load the external packages generally (ie putting them into the Description) - after all, most people will not use these external packages.

Any ideas how to just include an external package in the testing files but not in the DESCRIPTION?

Thanks!

Answer 1

I think you describe a very standard use of Suggests .

I see two related but separable issues:

• You want to test something using CI, in this case GHA. That is fine. Because you control the execution of the code, you could move your code from the test runner to, say, inst/examples and call it explicitly. That way the standard check of 'is the package using undeclared code' passes as inst/examples is not checked

• You want to not force other people to have to load these packages. That is fine too, and we have Suggests: for this! Read Section 1.1 of Writing R Extensions about all the detailed semantics. If your package invokes other packages via tests, the every R CMD check touches this (and the external packages) so they must be declared. But you already know that only "some" people will want to use this "some of the time": that is precisely what Suggests: does, and you bracket the use with if (requireNamespace(pkgHere, quietly=TRUE)) .

You can go either way, or even combine both. But you cannot call packages from tests and not declare them.