Whole file snapshot testing is designed for testing objects that don't have a convenient textual representation, with initial support for images (.png, .jpg, .svg), data frames (.csv), and text files (.R, .txt, .json, ...).

The first time expect_snapshot_file() is run, it will create _snaps/{test}/{name}.{ext} containing reference output. Future runs will be compared to this reference: if different, the test will fail and the new results will be saved in _snaps/{test}/{name}.new.{ext}. To review failures, call snapshot_review().

We generally expect this function to be used via a wrapper that takes care of ensuring that output is as reproducible as possible, e.g. automatically skipping tests where it's known that images can't be reproduced exactly.

  name = basename(path),
  binary = lifecycle::deprecated(),
  cran = FALSE,
  compare = compare_file_binary,
  variant = NULL

announce_snapshot_file(path, name = basename(path))

compare_file_binary(old, new)

compare_file_text(old, new)



Path to file to snapshot. Optional for announce_snapshot_file() if name is supplied.


Snapshot name, taken from path by default.


[Deprecated] Please use the compare argument instead.


Should these expectations be verified on CRAN? By default, they are not, because snapshot tests tend to be fragile because they often rely on minor details of dependencies.


A function used for comparison taking old and new arguments. By default this is compare_file_binary. Set it to compare_file_text to compare files line-by-line, ignoring the difference between Windows and Mac/Linux line endings.


If not-NULL, results will be saved in _snaps/{variant}/{test}/{name}.{ext}. This allows you to create different snapshots for different scenarios, like different operating systems or different R versions.

old, new

Paths to old and new snapshot files.

Announcing snapshots

testthat automatically detects dangling snapshots that have been written to the _snaps directory but which no longer have corresponding R code to generate them. These dangling files are automatically deleted so they don't clutter the snapshot directory. However we want to preserve snapshot files when the R code wasn't executed because of an unexpected error or because of a skip(). Let testthat know about these files by calling announce_snapshot_file() before expect_snapshot_file().


# To use expect_snapshot_file() you'll typically need to start by writing
# a helper function that creates a file from your code, returning a path
save_png <- function(code, width = 400, height = 400) {
  path <- tempfile(fileext = ".png")
  png(path, width = width, height = height)

path <- save_png(plot(1:5))
#> [1] "/tmp/RtmpoFHe0G/file32a729cd46fe.png"

if (FALSE) {
expect_snapshot_file(save_png(hist(mtcars$mpg)), "plot.png")

# You'd then also provide a helper that skips tests where you can't
# be sure of producing exactly the same output
expect_snapshot_plot <- function(name, code) {
  # Other packages might affect results
  skip_if_not_installed("ggplot2", "2.0.0")
  # Or maybe the output is different on some operation systems
  # You'll need to carefully think about and experiment with these skips

  name <- paste0(name, ".png")

  # Announce the file before touching `code`. This way, if `code`
  # unexpectedly fails or skips, testthat will not auto-delete the
  # corresponding snapshot file.
  announce_snapshot_file(name = name)

  path <- save_png(code)
  expect_snapshot_file(path, name)