Add transform argument to expect_snapshot_file() (#1530)

And tweak `compare` default so we don't imply a byte-by-byte comparison but  a line-by-line transformation.

Fixes #1474

Author: hadley

NEWS.md

@@ -1,5 +1,9 @@
 # testthat (development version)
 
+* `expect_snapshot_file()` gains a `transform` argument to match 
+  `expect_snapshot()` (#1474). `compare` now defaults to `NULL`, automatically 
+  guessing the comparison type based on the extension.
+
 * Snapshot cleanup also removes all empty directories (#1457).
 
 * `.new` file snapshots variants are no longer immediately deleted after

R/snapshot-file.R

@@ -21,9 +21,14 @@
 #' @param name Snapshot name, taken from `path` by default.
 #' @param binary `r lifecycle::badge("deprecated")` Please use the
 #'   `compare` argument instead.
-#' @param compare 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
+#' @param compare A function used to compare the snapshot files. It should take
+#'   two inputs, the paths to the `old` and `new` snapshot, and return either
+#'   `TRUE` or `FALSE`. This defaults to `compare_file_text` if `name` has
+#'   extension `.r`, `.R`, `.Rmd`, `.md`, or `.txt`, and otherwise uses
+#'   `compare_file_binary`.
+#'
+#'   `compare_file_binary()` compares byte-by-byte and
+#'   `compare_file_text()` compares lines-by-line, ignoring
 #'   the difference between Windows and Mac/Linux line endings.
 #' @param variant If not-`NULL`, results will be saved in
 #'   `_snaps/{variant}/{test}/{name}.{ext}`. This allows you to create
@@ -84,7 +89,8 @@ expect_snapshot_file <- function(path,
                                  name = basename(path),
                                  binary = lifecycle::deprecated(),
                                  cran = FALSE,
-                                 compare =  compare_file_binary,
+                                 compare = NULL,
+                                 transform = NULL,
                                  variant = NULL) {
   edition_require(3, "expect_snapshot_file()")
   if (!cran && !interactive() && on_cran()) {
@@ -107,6 +113,17 @@ expect_snapshot_file <- function(path,
     )
     compare <- if (binary) compare_file_binary else compare_file_text
   }
+  if (is.null(compare)) {
+    ext <- tools::file_ext(name)
+    is_text <- ext %in% c("r", "R", "txt", "md", "Rmd")
+    compare <- if (is_text) compare_file_text else compare_file_binary
+  }
+
+  if (!is.null(transform)) {
+    lines <- brio::read_lines(path)
+    lines <- transform(lines)
+    brio::write_lines(lines, path)
+  }
 
   lab <- quo_label(enquo(path))
   equal <- snapshotter$take_file_snapshot(name, path, compare, variant = variant)

man/expect_snapshot_file.Rd

@@ -0,0 +1,2 @@
+<redacted>
+ssh <redacted> squirrel

tests/testthat/_snaps/snapshot-file/secret.txt

@@ -81,6 +81,12 @@ test_that("can announce snapshot file", {
   expect_equal(snapper$snap_file_seen, "snapshot-announce/bar.svg")
 })
 
+test_that("can transform snapshot contents", {
+  path <- local_tempfile1(c("secret", "ssh secret squirrel"))
+
+  redact <- function(x) gsub("secret", "<redacted>", x)
+  expect_snapshot_file(path, "secret.txt", transform = redact)
+})
 
 # snapshot_file_equal -----------------------------------------------------