Stubbing out a function called within S3 dispatch

[nbenn]

I'm running into issues trying to mock a function with S3 dispatch involved. Consider the following setting:

gen <- function(x) UseMethod("gen")

met <- function(x) foo(x)

foo <- function(x) stop("foo")

.S3method("gen", "cls", met)

x <- structure(123, class = "cls")

gen(x)
#> Error in foo(x) : foo

I would now like to be able to call gen(x) is a test where foo() is stubbed out to avoid throwing the error. I assume this is currently not possible using mockery::stub()?

mockery::stub(gen, "foo", "bar", depth = 1)

gen(x)
#> Error in foo(x) : foo

mockery::stub(gen, "foo", "bar", depth = 2)

gen(x)
#> Error in foo(x) : foo

mockery::stub(gen, "foo", "bar", depth = 3)

gen(x)
#> Error in foo(x) : foo

# etc...

[jimhester]

This limitation doesn't really seem to cause many issues in practice, you usually would mock the generic rather than a method.

[nbenn]

Well ok, for whatever reason I was in the situation that I had written some tests using testthat::with_mock() in a scenario like

x <- structure(123, class = "cls")

testthat::with_mock(foo = function(x) "yay!", gen(x))
#> [1] "yay!"

Then I switched to testthat 3e and found that with the current state of things I was not able to do the above using the advertised alternatives. Fine for me if you say this is not in-scope for 'mockery'. I'll close then.

[MichaelChirico]

I just ran into this myself. It's not clear how to use mockery to replace with_mock here:

https://github.com/r-lib/lintr/blob/aafba478622c6f43c3fb329c69945c36a4d94b68/tests/testthat/test-ci.R#L47-L49

    with_mock(github_comment = function(x, ...) cat(x, "\n"), .env = asNamespace("lintr"), {
      expect_output(print(l), "*warning:*", fixed = TRUE)
    })

All of the following failed:

foo <- function(x, ...) cat(x, "\n")
mockery::stub(lintr:::print.lints, "github_comment", foo) # lintr:::github_comment executes
mockery::stub(lintr:::print.lints, "github_comment", foo, depth = 2) # error: cannot add bindings to a locked environment
mockery::stub(print, "github_comment", foo, depth = 2L) # weird error "use of NULL environment is defunct"
mockery::stub(print, "github_comment", foo) # lintr:::github_comment executes

[nbenn]

@MichaelChirico I started to roll my own, as I was not happy with what {mockery} and {mockr} offered in terms of functionality: {mockthat}. Has worked fine for my own use so far, but I'm sure there are edge-cases, that will break things for my approach too. Happy to take feedback/issues/etc.