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Introduction

Snapshot testing captures the output of a function or computation and saves it to a file. On subsequent test runs, the output is compared to the saved snapshot. If the output changes, the test fails — alerting you to unexpected behaviour.

snapr builds on top of testthat’s expect_snapshot_file() to provide convenient helpers for common R objects:

• expect_snapshot_data() — for data.frames (stores as CSV for human-readable diffs)
• expect_snapshot_object() — for any R object (stores as RDS, JSON, or plain text depending on the writer you choose)

Installation

Install the development version from GitHub:

# install.packages("pak")
pak::pak("d-morrison/snapr")

Basic Workflow

Snapshot tests live inside test_that() blocks, just like any other testthat expectation.

Snapshot a data.frame

Use expect_snapshot_data() when you want a human-readable CSV snapshot:

library(snapr)

test_that("iris subset is stable", {
  expect_snapshot_data(iris[1:5, ], name = "iris_head")
})

The first time you run this test, snapr creates tests/testthat/_snaps/iris_head.csv. On subsequent runs the data.frame is compared against that file and the test fails if anything has changed.

Snapshot any R object

Use expect_snapshot_object() for objects that cannot be easily represented as a CSV. The default writer (save_rds()) stores the object as an .rds file and uses diffobj to produce human-readable diffs during review:

test_that("model structure is stable", {
  model <- lm(mpg ~ wt, data = mtcars)
  expect_snapshot_object(model, name = "mpg_model")
})

Choosing a Writer

The writer argument of expect_snapshot_object() controls the file format. snapr ships four built-in writers:

JSON example

test_that("config list is stable", {
  config <- list(name = "my_app", version = "1.0.0", debug = TRUE)
  expect_snapshot_object(config, name = "config", writer = save_json)
})

Deparse example

test_that("simple list is stable", {
  expect_snapshot_object(list(x = 1:3), name = "simple", writer = save_deparse)
})

Handling Platform Differences

Some R objects (particularly RDS files and results from numerical algorithms) can differ across operating systems or R versions. snapr provides helper functions to create variant strings that are appended to snapshot names, keeping snapshots separate per platform:

test_that("RDS snapshot is platform-safe", {
  result <- list(x = rnorm(3))
  expect_snapshot_object(
    result,
    name = "result",
    variant = platform_variant()
  )
})

Text-based formats (JSON, CSV, deparse) typically produce identical output across platforms and do not require a variant.

Reviewing Snapshots

When a snapshot test fails, you can interactively review the differences with:

testthat::snapshot_review()

This opens a Shiny app that displays the old and new snapshots side-by-side, letting you accept or reject each change.

For richer, visual diffs when reviewing RDS snapshots, install the d-morrison/diffviewer fork of diffviewer. This fork wraps diffobj to compare deserialized R objects with arbitrary structures rather than raw bytes:

pak::pak("d-morrison/diffviewer")

To accept all new snapshots at once (e.g., after an intentional change):

testthat::snapshot_accept()

Summary

These binaries (installable software) and packages are in development.
They may not be fully stable and should be used with caution. We make no claims about them.
Health stats visible at Monitor.