Vignette on testing challenging functions

[maelle]

At the moment the only reference to mockery is via the deprecation message of former mocking functions.

Could there be a link to mockery maybe from the README?

[maelle]

https://blog.r-hub.io/2019/10/29/mocking/ was relatively popular (well for an R-hub post 🙂 ) so I think a vignette about such topics would be awesome.

[maelle]

Regarding plots, some guidance on testthat visual snapshots vs vdiffr (or how the two tools complement each other) would be nice.

[hadley]

• Use of expect_snapshot() for length-y text output
• Plus use of mocking and/or transform to eliminate spurious differences

[hadley]

I made this list for a recent workshop on testing:

• Output affected by RNG — withr::local_seed()
• Output affected by other external state — withr::local_options() / withr::local_envvar()
• Graphical output — https://vdiffr.r-lib.org/
• Errors & other user facing text — snapshots
• HTTP responses — httr2 mocking + httptest2
• Interactivity — mocking

Are there any other major classes of challenge that you can think of?

[maelle]

Regarding interactivity (and other examples like "code that should be run if there is no internet connection"), maybe also mention escape hatches? https://blog.r-hub.io/2023/01/23/code-switch-escape-hatch-test/

[hadley]

@maelle in that case (especially with local_mocked_bindings()), mocking feels like a clear win.

[maelle]

Interactivity — mocking

The mockery package supported returning multiple return values. We encountered this use case in a package under review at rOpenSci, where readline() is mocked.
My best guess is that you'd replace this behavior by using testthat's mocking + a function factory but I'm not sure.

[hadley]

@maelle testthat could maybe provide a helper for that case:

mock_sequence <- function(values) {
  force(values)
  i <- 1
  function(...) {
    value <- values[[i]]
    i <<- i + 1
    value
  } 
}

local_mocked_bindings(readline = mock_sequence(c("3", "This is a note", "n")))

[maelle]

Yes that would be helpful 😸 Happy to make a PR if relevant.
Yesterday I tried looking for use cases of mockery's "multiple return values" but it's not easy to search for 😅

[hadley]

A PR would be great. I think the main challenge is what to call it. Maybe it's reasonable to have a new series of functions that start with mock?

Probably also need to error if i > length(values).

[maelle]

mock_output_sequence()?

It reminds me of an use case in HTTP testing, where one mocks an API returning a 503 then a 200 status code for instance. It works with vcr or webmockr but it doesn't really have a name there.

I'll make a PR soon-ish.

[maelle]

#2061