9.1 Setup

Before working on all this, we need to install presser.

Then, we need to add presser as a Suggests dependency of our package. At the time of writing we are using a handy function available only in presser above version 1.1.0.9000, which means installing presser from GitHub and not CRAN, so we have to run remotes::install_github("r-lib/presser") and then usethis::use_dev_package("presser", type = "Suggests").

Last but not least, we create a setup file at tests/testthat/setup.R. When testthat runs tests, files whose name starts with “setup” are always run first. We need to ensure that we set up a fake API key when there is no API token around. Why? Because if you remember well, the code of our function gh_organizations() checks for the presence of a token. When using our own fake web service, we obviously don’t really need a token but we still need to fool our own package in contexts where there is no token (e.g. in continuous integration checks for a fork of a GitHub repository).

if(!nzchar(Sys.getenv("REAL_REQUESTS"))) {
  Sys.setenv("GITHUB_PAT" = "foobar")
}

The setup file could also load presser, but in our demo we will namespace presser functions instead.

9.2 Actual testing

With presser we will be spinning local fake web services that we will want our package to interact with instead of the real APIs. Therefore, we first need to amend the code of functions returning URLs to services to be able to change them via an environment variable. They become:

status_url <- function() {

  env_url <- Sys.getenv("EXEMPLIGHRATIA_GITHUB_STATUS_URL")

  if (nzchar(env_url)) {
    return(env_url)
  }

  "https://kctbh9vrtdwd.statuspage.io/api/v2/components.json"
}

and

gh_v3_url <- function() {

  api_url <- Sys.getenv("EXEMPLIGHRATIA_GITHUB_API_URL")

  if (nzchar(api_url)) {
    return(api_url)
  }

  "https://api.github.com/"
}

Having these two switches is crucial.

Then, let’s tweak our test of gh_api_status().

test_that("gh_api_status() works", {
  if (!nzchar(Sys.getenv("REAL_REQUESTS"))) {
   app <- presser::new_app()
      app$get("/", function(req, res) {
        res$send_json(
          list( components =
          list(
            list(
            name = "API Requests",
            status = "operational"
            )
          )
          ),
          auto_unbox = TRUE
        )
      })
    web <-  presser::local_app_process(app, start = TRUE)
    web$local_env(list(EXEMPLIGHRATIA_GITHUB_STATUS_URL = "{url}"))
  }

  testthat::expect_type(gh_api_status(), "character")
})

So what’s happening here?

• When we’re not asking for requests to the real service to be made (Sys.getenv("REAL_REQUESTS")), we prepare a new app via presser::new_app(). It’s a very simple one, that returns, for GET requests, a list corresponding to what we’re used to getting out of the status API, except that a) it’s much smaller and b) the “operational” status is hard-coded.
• When then create a local app process via presser::local_app_process(, start = TRUE). It will start right away thanks to start=TRUE but we could have chosen to start it later via calling e.g. web$url() (see ?presser::local_app_process); and most importantly it will be stopped automatically after the test. No mess made!
• We set the EXEMPLIGHRATIA_GITHUB_STATUS_URL variable to the URL of the local app process. This is what connects our code to our fake web service.

It might seem like a lot of overhead code but

• It means no real requests are made which is our ultimate goal.
• We will get used to it.
• We can write helper code in a testthat helper file to not repeat ourselves in further test files; there could even be an app shared between all test files depending on your package.

Now, let’s add a test for error behavior. This inspired us to change error behavior a bit with a slightly more specific error message i.e. httr::stop_for_status(response) became httr::stop_for_status(response, task = "get API status, ouch!").

test_that("gh_api_status() errors when the API does not behave", {
  app <- presser::new_app()
  app$get("/", function(req, res) {
    res$send_status(502L)
  })
  web <-  presser::local_app_process(app, start = TRUE)
  web$local_env(list(EXEMPLIGHRATIA_GITHUB_STATUS_URL = "{url}"))
  testthat::expect_error(gh_api_status(), "ouch")
})

It’s a similar process to the earlier test:

• setting up a new app;
• having it return something we chose, in this case a 502 status;
• launching a local app process;
• connecting our code to it via setting the EXEMPLIGHRATIA_GITHUB_STATUS_URL environment variable to the URL of the fake service;
• test.

Last but not least let’s convert our test of gh_organizations(),

test_that("gh_organizations works", {

  if (!nzchar(Sys.getenv("REAL_REQUESTS"))) {
    app <- presser::new_app()
    app$get("/organizations", function(req, res) {
      res$send_json(
        jsonlite::read_json(
          testthat::test_path(
            file.path("responses", "organizations.json")
          )
        ),
        auto_unbox = TRUE
        )
    })
    web <-  presser::local_app_process(app, start = TRUE)
    web$local_env(list(EXEMPLIGHRATIA_GITHUB_API_URL = "{url}"))
  }

  testthat::expect_type(gh_organizations(), "character")
})

As before we

• create a new app;
• have it returned something we chose for a GET request of the /organizations endpoint. In this case, we have it return the content of a JSON file we created at tests/testthat/responses/organizations.json by copy-pasting a real response from the API;
• launch a local app process;
• set its URL as the EXEMPLIGHRATIA_GITHUB_API_URL environment variable;
• test.

9.3 Also testing for real interactions

What if the API responses change? Hopefully we’d notice that thanks to following API news. However, sometimes web APIs change without any notice. Therefore it is important to run tests against the real web service once in a while.

In our tests we have used the condition

if (!nzchar(Sys.getenv("REAL_REQUESTS"))) {

before launching the app and using its URL as URL for the service. So if our tests are generic enough, we can add a CI build where the environment variable REAL_REQUESTS is set to true. If they are not generic enough, we can use theapproach exemplified in the chapter about httptest.

• set up a folder real-tests with tests interacting with the real web service;
• add it to Rbuildignore;
• in a CI build, delete tests/testthat and replace it with real-tests, before running R CMD check.

9.4 Summary

• We set up presser usage in our package exemplighratia by adding a dependency on presser and by adding a setup file to fool our own package that needs an API token.
• We created and launched fake apps in our test files.

Now, how do we make sure this works?

• Turn off wifi, run the tests again. It works! Turn on wifi again.
• Open .Renviron (usethis::edit_r_environ()), edit “GITHUB_PAT” into “byeGITHUB_PAT,” re-start R, run the tests again. It works! Fix your “GITHUB_PAT” token in .Renviron.

So we now have tests that no longer rely on an internet connection nor on having API credentials.

For the full list of changes applied to exemplighratia in this chapter, see the pull request diff on GitHub.