Testing R Packages

Testing R Packages with testthat

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

Surveyverse projects: This skill covers generic testthat patterns. Project decisions — including the ban on describe() blocks (flat test_that() only), coverage targets, # nocov policy, and error testing patterns — are in .claude/rules/testing-standards.md. Package-specific conventions (test_invariants(), data generators, numerical tolerances) are in .claude/rules/testing-{package}.md. Those rules take precedence over generic guidance here, including any examples in this skill that use describe().

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 <- my_function(input)
  expect_equal(result, expected_value)
})

Test descriptions should read naturally and describe behavior, not implementation.

BDD Syntax (describe/it)

For behavior-driven development, use describe() and it():

describe("matrix()", {
  it("can be multiplied by a scalar", {
    m1 <- matrix(1:4, 2, 2)
    m2 <- m1 * 2
    expect_equal(matrix(1:4 * 2, 2, 2), m2)
  })

  it("can be transposed", {
    m <- matrix(1:4, 2, 2)
    expect_equal(t(m), matrix(c(1, 3, 2, 4), 2, 2))
  })
})

Key features:

• describe() groups related specifications for a component
• it() defines individual specifications (like test_that())
• Supports nesting for hierarchical organization
• it() without code creates pending test placeholders

Use describe() to verify you implement the right things, use test_that() to ensure you do things right.

See references/bdd.md for comprehensive BDD patterns, nested specifications, and test-first workflows.

Running Tests

Three scales of testing:

Micro (interactive development):

devtools::load_all()
expect_equal(foofy(...), expected)

Mezzo (single file):

testthat::test_file("tests/testthat/test-foofy.R")
# RStudio: Ctrl/Cmd + Shift + T

Macro (full suite):

devtools::test()    # Ctrl/Cmd + Shift + T
devtools::check()   # Ctrl/Cmd + Shift + E

Core Expectations

Equality

expect_equal(10, 10 + 1e-7)      # Allows numeric tolerance
expect_identical(10L, 10L)       # Exact match required
expect_all_equal(x, expected)    # Every element matches (v3.3.0+)

Errors, Warnings, Messages

expect_error(1 / "a")
expect_error(bad_call(), class = "specific_error_class")
expect_no_error(valid_call())

expect_warning(deprecated_func())
expect_no_warning(safe_func())

expect_message(informative_func())
expect_no_message(quiet_func())

Pattern Matching

expect_match("Testing is fun!", "Testing")
expect_match(text, "pattern", ignore.case = TRUE)

Structure and Type

expect_length(vector, 10)
expect_type(obj, "list")
expect_s3_class(model, "lm")
expect_s4_class(obj, "MyS4Class")
expect_r6_class(obj, "MyR6Class")      # v3.3.0+
expect_shape(matrix, c(10, 5))         # v3.3.0+

Sets and Collections

expect_setequal(x, y)           # Same elements, any order
expect_contains(fruits, "apple") # Subset check (v3.2.0+)
expect_in("apple", fruits)       # Element in set (v3.2.0+)
expect_disjoint(set1, set2)      # No overlap (v3.3.0+)

Logical

expect_true(condition)
expect_false(condition)
expect_all_true(vector > 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 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 for detailed fixture patterns.

Mocking

Replace external dependencies during testing using local_mocked_bindings(). See 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

See references/patterns.md for examples: error classes, temporary files, modified options, multiple cases, custom expectations, file system discipline, and testthat 3 modernizations.

Advanced Topics

For advanced testing scenarios, see:

• references/bdd.md - BDD-style testing with describe/it, nested specifications, test-first workflows
• references/snapshots.md - Snapshot testing, transforms, variants
• references/mocking.md - Mocking strategies, webfakes, httptest2
• references/fixtures.md - Fixture patterns, database fixtures, helper files
• references/patterns.md - Common patterns, file system discipline, testthat 3 modernizations
• references/advanced.md - Skipping tests, secrets management, CRAN requirements, custom expectations, parallel testing

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)