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Imagine you have a function that does something complicated, and in the middle of its definition it generates a variable. Now suppose that you want to save this variable and then re-use it for tests, what I mean is that you want your function to always reproduce this intermediary variable, regardless of what you give it as inputs. This can be useful for testing, if computing this intermediate variable is costly.

In my {rix} package, the rix() function generates valid Nix expressions from R input and these Nix expressions can then be used to build reproducible development environments that include R, R packages, development libraries, and so on. If you want a 5-minute intro to {rix}, click here.

Anyways, sometimes, computing these expressions can take some time, especially if the users wants to include remote dependencies that have themselves remote dependencies. rix() will try to look for suitable GitHub commits to pin all the packages for reproducibility purposes, and this can imply quite a lot of api calls. Now for my tests, I wanted to use an already generated default.nix file (which contains the generated Nix expression) but I didn’t want to have to recompute it every time I ran the test and I couldn’t simply use it as is for the test either. You see, that default.nix was in an intermediary state, before rix() is supposed to do some post-processing to it, which is what I actually want to test (I want to actually test the argument that makes rix() skip this post-processing step).

So suppose rix() looks like this:

rix <- function(a,b,c){
  ... # lots of code
  ... # lots of code
  default.nix_file <- ... # it's generated here
  # Then a bunch of things happen to it
  out <- f(default.nix_file)
  writeLines(out, path) # this is what's written
}

Now what I want is to be able to “overwrite” the default.nix_file variable on line 4 when testing, to provide want I want. This way, I can call rix() with some “easy” parameters that make the computations up to that point very quick. My goal is essentially to test f() (line 6), which begs the question, why not write f() as a separate function and test it? This would be the best practice, however, I don’t really have such an f(), rather it’s a series of complicated steps that follow and rewriting everything to make it easily testable would just take too much time.

Instead, I opted for the following:

rix <- function(a,b,c){
  ... # lots of code
  ... # lots of code

  stub_default.nix <- getOption("TESTTHAT_DEFAULT.NIX", default = NULL)

  if(!is.null(stub_default.nix)){
    default.nix_file <- readLines(stub_default.nix)
  } else {
    default.nix_file <- ... # it's generated here if not being tested
  }
  out <- f(default.nix_file)
  # Then a bunch of things happen to it
  writeLines(out, path) # this is what's written
}

On line 5, I get the option "TESTTHAT_DEFAULT.NIX" and if it doesn’t exist, stub_default.nix will be set to NULL. So if it’s NULL it’s business as usual, if not, then that default.nix file dedicated for testing gets passed further down. In a sense, I injected the variable I needed in the spot I needed.

Then, my tests looks like this:

testthat::test_that("remove_duplicate_entries(), don't remove duplicates if skip", {


  dups_entries_default.nix <- paste0(
    testthat::test_path(),
    "/testdata/default-nix_samples/dups-entries_default.nix")

  tmpdir <- tempdir()

  # This copies the file I need in the right path
  destination_file <- file.path(tempdir(), basename(dups_entries_default.nix))
  file.copy(dups_entries_default.nix, destination_file, overwrite = TRUE)

  on.exit(
    unlink(tmpdir, recursive = TRUE, force = TRUE),
    add = TRUE
  )

  removed_dups <- function(destination_file) {

    # Set the option to the file path and clean the option afterwards
    op <- options("TESTTHAT_DEFAULT.NIX" = destination_file)
    on.exit(options(op), add = TRUE, after = FALSE)

    out <- rix(
      date = "2025-02-10",
      project_path = tmpdir,
      overwrite = TRUE,
      skip_post_processing = TRUE) # <- this is actually want I wanted to test
    file.path(destination_file)
  }


  testthat::expect_snapshot_file(
    path = removed_dups(destination_file),
    name = "skip-dups-entries_default.nix",
  )
})

On line 22, I set the option and on line 23 I write code to remove that option once the test is done, to not mess up subsequent tests. This is a snapshot test, so now I can take a look at the resulting file, and indeed make sure that post-processing was skipped, as expected.

How would you have done this?