Testing R Packages

Testing R Packages with testthat

Modern best practices for R package testing using testthat 3+.

Initial Setup

Initialize testing with testthat 3rd edition:

usethis::use_testthat(3)

This creates tests/testthat/ directory, adds testthat to DESCRIPTION Suggests with Config/testthat/edition: 3, and creates tests/testthat.R.

File Organization

Mirror package structure:

• Code in R/foofy.R → tests in tests/testthat/test-foofy.R
• Use usethis::use_r("foofy") and usethis::use_test("foofy") to create paired files

Special files:

• helper-*.R - Helper functions and custom expectations, sourced before tests
• setup-*.R - Run during R CMD check only, not during load_all()
• fixtures/ - Static test data files accessed via test_path()

Test Structure

Tests follow a three-level hierarchy: File → Test → Expectation

Standard Syntax

test_that("descriptive behavior", {
  result  0)      # All elements TRUE (v3.3.0+)
expect_all_false(vector < 0)     # All elements FALSE (v3.3.0+)

Design Principles

1. Self-Sufficient Tests

Each test should contain all setup, execution, and teardown code:

# Good: self-contained
test_that("foofy() works", {
  data <- data.frame(x = 1:3, y = letters[1:3])
  result <- foofy(data)
  expect_equal(result$x, 1:3)
})

# Bad: relies on ambient state
dat <- data.frame(x = 1:3, y = letters[1:3])
test_that("foofy() works", {
  result <- foofy(dat)  # Where did 'dat' come from?
  expect_equal(result$x, 1:3)
})

2. Self-Contained Tests (Cleanup Side Effects)

Use withr to manage state changes:

test_that("function respects options", {
  withr::local_options(my_option = "test_value")
  withr::local_envvar(MY_VAR = "test")
  withr::local_package("jsonlite")

  result <- my_function()
  expect_equal(result$setting, "test_value")
  # Automatic cleanup after test
})

Common withr functions:

• local_options() - Temporarily set options
• local_envvar() - Temporarily set environment variables
• local_tempfile() - Create temp file with automatic cleanup
• local_tempdir() - Create temp directory with automatic cleanup
• local_package() - Temporarily attach package

3. Plan for Test Failure

Write tests assuming they will fail and need debugging:

• Tests should run independently in fresh R sessions
• Avoid hidden dependencies on earlier tests
• Make test logic explicit and obvious

4. Repetition is Acceptable

Repeat setup code in tests rather than factoring it out. Test clarity is more important than avoiding duplication.

5. Use devtools::load_all() Workflow

During development:

• Use devtools::load_all() instead of library()
• Makes all functions available (including unexported)
• Automatically attaches testthat
• Eliminates need for library() calls in tests

Snapshot Testing

For complex output that's difficult to verify programmatically, use snapshot tests. See [references/snapshots.md](references/snapshots.md) for complete guide.

Basic pattern:

test_that("error message is helpful", {
  expect_snapshot(
    error = TRUE,
    validate_input(NULL)
  )
})

Snapshots stored in tests/testthat/_snaps/.

Workflow:

devtools::test()                    # Creates new snapshots
testthat::snapshot_review('name')   # Review changes
testthat::snapshot_accept('name')   # Accept changes

Test Fixtures and Data

Three approaches for test data:

1. Constructor functions - Create data on-demand:

new_sample_data <- function(n = 10) {
  data.frame(id = seq_len(n), value = rnorm(n))
}

2. Local functions with cleanup - Handle side effects:

local_temp_csv <- function(data, env = parent.frame()) {
  path <- withr::local_tempfile(fileext = ".csv", .local_envir = env)
  write.csv(data, path, row.names = FALSE)
  path
}

3. Static fixture files - Store in fixtures/ directory:

data <- readRDS(test_path("fixtures", "sample_data.rds"))

See [references/fixtures.md](references/fixtures.md) for detailed fixture patterns.

Mocking

Replace external dependencies during testing using local_mocked_bindings(). See [references/mocking.md](references/mocking.md) for comprehensive mocking strategies.

Basic pattern:

test_that("function works with mocked dependency", {
  local_mocked_bindings(
    external_api = function(...) list(status = "success", data = "mocked")
  )

  result <- my_function_that_calls_api()
  expect_equal(result$status, "success")
})

Common Patterns

Testing Errors with Specific Classes

test_that("validation catches errors", {
  expect_error(
    validate_input("wrong_type"),
    class = "vctrs_error_cast"
  )
})

Testing with Temporary Files

test_that("file processing works", {
  temp_file <- withr::local_tempfile(
    lines = c("line1", "line2", "line3")
  )

  result <- process_file(temp_file)
  expect_equal(length(result), 3)
})

Testing with Modified Options

test_that("output respects width", {
  withr::local_options(width = 40)

  output <- capture_output(print(my_object))
  expect_lte(max(nchar(strsplit(output, "\n")[[1]])), 40)
})

Testing Multiple Related Cases

test_that("str_trunc() handles all directions", {
  trunc <- function(direction) {
    str_trunc("This string is moderately long", direction, width = 20)
  }

  expect_equal(trunc("right"), "This string is mo...")
  expect_equal(trunc("left"), "...erately long")
  expect_equal(trunc("center"), "This stri...ely long")
})

Custom Expectations in Helper Files

# In tests/testthat/helper-expectations.R
expect_valid_user <- function(user) {
  expect_type(user, "list")
  expect_named(user, c("id", "name", "email"))
  expect_type(user$id, "integer")
  expect_match(user$email, "@")
}

# In test file
test_that("user creation works", {
  user <- create_user("test@example.com")
  expect_valid_user(user)
})

File System Discipline

Always write to temp directory:

# Good
output <- withr::local_tempfile(fileext = ".csv")
write.csv(data, output)

# Bad - writes to package directory
write.csv(data, "output.csv")

Access test fixtures with test_path():

# Good - works in all contexts
data <- readRDS(test_path("fixtures", "data.rds"))

# Bad - relative paths break
data <- readRDS("fixtures/data.rds")

Advanced Topics

For advanced testing scenarios, see:

• [references/bdd.md](references/bdd.md) - BDD-style testing with describe/it, nested specifications, test-first workflows
• [references/snapshots.md](references/snapshots.md) - Snapshot testing, transforms, variants
• [references/mocking.md](references/mocking.md) - Mocking strategies, webfakes, httptest2
• [references/fixtures.md](references/fixtures.md) - Fixture patterns, database fixtures, helper files
• [references/advanced.md](references/advanced.md) - Skipping tests, secrets management, CRAN requirements, custom expectations, parallel testing

testthat 3 Modernizations

When working with testthat 3 code, prefer modern patterns:

Deprecated → Modern:

• context() → Remove (duplicates filename)
• expect_equivalent() → expect_equal(ignore_attr = TRUE)
• with_mock() → local_mocked_bindings()
• is_null(), is_true(), is_false() → expect_null(), expect_true(), expect_false()

New in testthat 3:

• Edition system (Config/testthat/edition: 3)
• Improved snapshot testing
• waldo::compare() for better diff output
• Unified condition handling
• local_mocked_bindings() works with byte-compiled code
• Parallel test execution support

Quick Reference

Initialize: usethis::use_testthat(3)

Run tests: devtools::test() or Ctrl/Cmd + Shift + T

Create test file: usethis::use_test("name")

Review snapshots: testthat::snapshot_review()

Accept snapshots: testthat::snapshot_accept()

Find slow tests: devtools::test(reporter = "slow")

Shuffle tests: devtools::test(shuffle = TRUE)

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: posit-dev
• Source: posit-dev/skills
• License: MIT

Install and usage instructions live in the source repository linked above.