| expect_snapshot_file {testthat} | R Documentation |
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)
path |
Path to file to snapshot. |
name |
Snapshot name, taken from |
binary |
If |
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. Even on CRAN, the
expectations will still fail if |
# 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
## Not run:
expect_snapshot_file(save_png(hist(mtcars)), "plot.png")
## End(Not run)
# You'd then also provide a helper that skip 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"))
}