Snapshot testing — expect_snapshot • testthat

Snapshot tests (aka golden tests) are similar to unit tests except that the expected result is stored in a separate file that is managed by testthat. Snapshot tests are useful for when the expected value is large, or when the intent of the code is something that can only be verified by a human (e.g. this is a useful error message). Learn more in vignette("snapshotting").

expect_snapshot() captures all messages, warnings, errors, and output from code.
expect_snapshot_output() captures just output printed to the console.
expect_snapshot_error() captures an error message and optionally checks its class.
expect_snapshot_warning() captures a warning message and optionally checks its class.
expect_snapshot_value() captures the return value.

(These functions supersede verify_output(), expect_known_output(), expect_known_value(), and expect_known_hash().)

Usage

expect_snapshot(
  x,
  cran = FALSE,
  error = FALSE,
  transform = NULL,
  variant = NULL,
  cnd_class = FALSE
)

expect_snapshot_output(x, cran = FALSE, variant = NULL)

expect_snapshot_error(x, class = "error", cran = FALSE, variant = NULL)

expect_snapshot_warning(x, class = "warning", cran = FALSE, variant = NULL)

expect_snapshot_value(
  x,
  style = c("json", "json2", "deparse", "serialize"),
  cran = FALSE,
  tolerance = testthat_tolerance(),
  ...,
  variant = NULL
)

Arguments

x

Code to evaluate.

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.

error

Do you expect the code to throw an error? The expectation will fail (even on CRAN) if an unexpected error is thrown or the expected error is not thrown.

transform

Optionally, a function to scrub sensitive or stochastic text from the output. Should take a character vector of lines as input and return a modified character vector as output.

variant

If not-NULL, results will be saved in _snaps/{variant}/{test.md}, so variant must be a single string of alphanumeric characters suitable for use as a directory name.

You can variants to deal with cases where the snapshot output varies and you want to capture and test the variations. Common use cases include variations for operating system, R version, or version of key dependency. Variants are an advanced feature. When you use them, you'll need to carefully think about your testing strategy to ensure that all important variants are covered by automated tests, and ensure that you have a way to get snapshot changes out of your CI system and back into the repo.

cnd_class

Whether to include the class of messages, warnings, and errors in the snapshot. Only the most specific class is included, i.e. the first element of class(cnd).

class

Class of expected error or warning. The expectation will always fail (even on CRAN) if an error of this class isn't seen when executing x.

style

Serialization style to use:

json uses jsonlite::fromJSON() and jsonlite::toJSON(). This produces the simplest output but only works for relatively simple objects.
json2 uses jsonlite::serializeJSON() and jsonlite::unserializeJSON() which are more verbose but work for a wider range of type.
deparse uses deparse(), which generates a depiction of the object using R code.
serialize() produces a binary serialization of the object using serialize(). This is all but guaranteed to work for any R object, but produces a completely opaque serialization.

tolerance

Numerical tolerance: any differences (in the sense of base::all.equal()) smaller than this value will be ignored.

The default tolerance is sqrt(.Machine$double.eps), unless long doubles are not available, in which case the test is skipped.

...

For expect_snapshot_value() only, passed on to waldo::compare() so you can control the details of the comparison.

Workflow

The first time that you run a snapshot expectation it will run x, capture the results, and record in tests/testthat/snap/{test}.json. Each test file gets its own snapshot file, e.g. test-foo.R will get snap/foo.json.

It's important to review the JSON files and commit them to git. They are designed to be human readable, and you should always review new additions to ensure that the salient information has been captured. They should also be carefully reviewed in pull requests, to make sure that snapshots have updated in the expected way.

On subsequent runs, the result of x will be compared to the value stored on disk. If it's different, the expectation will fail, and a new file snap/{test}.new.json will be created. If the change was deliberate, you can approve the change with snapshot_accept() and then the tests will pass the next time you run them.

Note that snapshotting can only work when executing a complete test file (with test_file(), test_dir(), or friends) because there's otherwise no way to figure out the snapshot path. If you run snapshot tests interactively, they'll just display the current value.