Capture warnings and assign a value in one go?

Question

I am running some unit tests using the testthat package. I have a function that is tested repeatedly under various conditions. My problem is that the function has three kinds of "outputs":

1. Prints to the console. This should be discarded.
2. Elicits a warning sometimes. The test should pass on a particular warning, but fail for others.
3. Returns a value. I want to save that for further testing.

Is there a way to do this? I cannot change the function in question as it ships with another package.

Here is a simple repex:

func = function() {
  print("a print")
  warning("a warning")
  return("a value")
}

I can accomplish (1) and (3) by doing

capture.output(result <<- func())
# result can be tested here, but the warning is not captured

I can do (1) and (2) by doing

msg = testthat::capture_warning(capture.output(result <<- func()))
if (!is.null(msg$message)) {
   testthat::expect_true(msg$message == "a warning")
}
# result is undefined here

Again, is there a way to achieve all three goals above?

Answer 1

with quietly from the purrr package you can capture warnings, prints, messages, output while still returning a value.

Then, you may build a wrapping function around that to develop your checks and your conditions.

func <- function() {
  print("a print")
  warning("a warning")
  return("a value")
}

library(purrr)

res <- quietly(func)()
res

# $result
# [1] "a value"
#
# $output
# [1] "[1] \"a print\""
#
# $warnings
# [1] "a warning"
#
# $messages
# character(0)

quietly is a sort of function decorator.