Function Mocking for Unit Testing.

mockthat

With version 3.0.0 of testthat, mocking capabilities provided by testthat::with_mock() and testthat::local_mock() have been deprecated under edition 3. This leaves implementation of function mocking for unit testing to third-party packages, of which two have been published on CRAN: mockery and mockr. While all currently available mocking implementations have their limitations, what sets mockthat apart from mockery and mockr is coping with S3 dispatch (see example below).

Installation

You can install the development version of mockthat from GitHub with:

# install.packages("devtools")
devtools::install_github("nbenn/mockthat")

A release version will be submitted to CRAN shortly.

Example

Mocking in the context of unit testing refers to temporarily replacing a piece of functionality (that might be part of the package being tested or potentially even part of a downstream dependency) in order to cope with limited infrastructure in testing environments (for example absence of a live Internet connection).

For a function download_data() implemented as

download_data <- function(url) {
  curl::curl_fetch_memory(url)
}

we do not want to have to rely on a live internet connection for writing a unit test. With help of mockthat, we can substitute curl::curl_fetch_memory() with a stub that simply returns a constant.

library(mockthat)

url <- "https://eu.httpbin.org/get?foo=123"

with_mock(
  `curl::curl_fetch_memory` = function(...) '["mocked request"]',
  download_data(url)
)
#> [1] "[\"mocked request\"]"

As mentioned above, the main point of differentiation of mockthat over the other available packages mockery and mockr is stubbing out functions in the context of S3 dispatch. Assuming the following set-up,

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

mockthat::with_mock() can be used to catch the call to foo() and therefore prevent the error from being thrown.

mockthat::with_mock(
  foo = function(x) "bar",
  met(x)
)
#> [1] "bar"

This is not possible with the current implementation of mockr::with_mock().

mockr::with_mock(
  foo = function(x) "bar",
  met(x)
)
#> Warning: Replacing functions in evaluation environment: `foo()`
#> Warning: The code passed to `with_mock()` must be a braced expression to get
#> accurate file-line information for failures.
#> [1] "bar"

And with the current API of mockery::stub() it is unclear how the depth argument should be chosen, as the function gen() does not contain a call to met(). Trying a range of sensible values does not yield the desired result.

for (depth in seq_len(3L)) {
  mockery::stub(gen, "foo", "bar", depth = depth)
  tryCatch(met(x), error = function(e) message("depth ", depth, ": nope"))
}
#> depth 1: nope
#> depth 2: nope
#> depth 3: nope

Borrowing from mockery, mockthat also allows for creating mock objects (with class attribute mock_fun), which allow capture of the call for later examination.

mk <- mock("mocked request")
dl <- function(url) curl::curl(url)

with_mock(`curl::curl` = mk, dl(url))
#> [1] "mocked request"

mock_call(mk)
#> curl::curl(url = url)
mock_args(mk)
#> $url
#> [1] "https://eu.httpbin.org/get?foo=123"
#> 
#> $open
#> [1] ""
#> 
#> $handle
#> <curl handle> (empty)

In addition to with_mock(), mockthat also offers a local_mock() function, again, mimicking the deprecated testthat function, which keeps the mocks in place for the life-time of the environment passed as local_env argument (or if called from the global environment, until withr::deferred_run() is executed). Mock objects as shown above are created (and returned invisibly) for all non-function objects passed as ....

tmp <- new.env()
mk <- local_mock(`curl::curl` = "mocked request", local_env = tmp)
dl(url)
#> [1] "mocked request"
mock_arg(mk, "url")
#> [1] "https://eu.httpbin.org/get?foo=123"