Snapshot testing for whole files

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.

expect_snapshot_file(path, name = basename(path), binary = TRUE, cran = FALSE)

Arguments

path	Path to file to snapshot.
name	Snapshot name, taken from path by default.
binary	If FALSE, files are compared line-by-line, ignoring the difference between Windows and Mac/Linux line endings.
cran	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.

Examples

# 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)
  on.exit(dev.off())
  code

  path
}
path <- save_png(plot(1:5))
path
#> [1] "/tmp/Rtmpi6p2Gl/file67694f42902a.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
  skip_on_os("windows")
  # You'll need to carefully think about and experiment with these skips

  path <- save_png(code)
  expect_snapshot_file(path, paste0(name, ".png"))
}