For complex printed output and objects, it is often challenging to describe exactly what you expect to see. expect_known_value() and expect_known_output() provide a slightly weaker guarantee, simply asserting that the values have no changed since the last time that you ran them.

expect_known_output(object, file, update = TRUE, ..., info = NULL,
  label = NULL, print = FALSE, width = 80)

expect_known_value(object, file, update = TRUE, ..., info = NULL,
  label = NULL)

expect_known_hash(object, hash = NULL)



object to test


File path where known value/output will be stored.


Should the file be updated? Defaults to TRUE, with the expectation that you'll notice changes because of the first failure, and then see the modified files in git.


other values passed to all.equal() or [identical()].


extra information to be included in the message (useful when writing tests in loops).


object label. When NULL, computed from deparsed object.


If TRUE and the result of evaluating code is visible this will print the result, ensuring that the output of printing the object is included in the overall output


Number of characters per line of output


Known hash value. Leave empty and you'll be informed what to use in the test output.


These expectations should be used in conjunction with git, as otherwise there is no way to revert to previous values. Git is particularly useful in conjunction with expect_known_output() as the diffs will show you exactly what has changed.

Note that known values updates will only updated when running tests interactively. R CMD check clones the package source so any changes to the reference files will occured a temporary directory, and will not be synchronised back to the source package.


tmp <- tempfile() # The first run always succeeds expect_known_output(mtcars[1:10, ], tmp, print = TRUE)
#> Warning: Creating reference output
# Subsequent runs will suceed only if the file is unchanged # This will succeed: expect_known_output(mtcars[1:10, ], tmp, print = TRUE)
# NOT RUN { # This will fail expect_known_output(mtcars[1:9, ], tmp, print = TRUE) # }