Test package function that writes to disk

Question

I am trying to write a test for a package function in R.

Let's say we have a function that simply writes a string x to disk using writeLines():

exporting_function <- function(x, file) {

 writeLines(x, con = file)

 invisible(NULL)
}

One way of testing it would be to check if a file exists. Typically, it should not exist at first, but after the exporting function was run it should. Also, you might want to test the file size to be greater than 0:

library(testthat)

test_that("file is written to disk", {
 file = 'output.txt'
 expect_false(file.exists(file))

 exporting_function("This is a test",
                    file = file)


 expect_true(file.exists(file))

 expect_gt(file.info('output.txt')$size, 0)
})

Is this a good way to test it? In the CRAN Repository Policy it states that Packages should not write in the user’s home filespace (including clipboards), nor anywhere else on the file system apart from the R session’s temporary directory. Would this test violate this constraint?

There is a expect_output_file function. From the documentation and examples I am not sure if this is a more appropriate expectation to test the function. It requires a.o. an object argument which should be the object to test. What is the object to test in my case?

Redraft

The policy also states "Limited exceptions may be allowed in interactive sessions if the package obtains confirmation from the user." I have to assume that includes implied confirmation, such as from using functions whose explicit purpose is to write to disk; otherwise packages such as devtools, Rcpp, and RcppArmadillo could never be on CRAN since they set up R package directory structures for users. – Houseboy

The part of your question asking about how to test the function is a good one for Stack Overflow. The part asking about CRAN policy might be better suited for the R-package-devel mailing list. – Houseboy

it would be also great to add to the question: how to properly add example to the function that writes to the file – Bilbao

That can be done using ` \dontrun{}` – Redraft

Answer 1

That looks as if it violates CRAN policy. Why not simply write to the temporary directory, using

file <-  tempfile()

in place of

file = 'output.txt'

?

As to whether it is a good test: wouldn't it be better to try reading the file back in, and confirming that what was read matches what was written? That's easy in your toy example. It might be harder in the real one, but having an import function paired with your export function is always a good idea.

Multinuclear

Good point, the actual file will be a PNG, JPEG, or PDF, which makes it harder to test. – Redraft

you will also need to clean up the temp file afterwards, and avoid a failing test stopping the clean up. i had this issue with a CRAN package, and CRAN didn't like it. – Hutt

@JackWasey seems like a on.exit(unlink(file)) would satisfy that. – Ongoing