testing-patterns-with-testthat

name	Testing Patterns with testthat
description	Comprehensive guide to testing R packages using testthat 3rd edition, including test structure, expectations, fixtures, and snapshot testing

Testing Patterns with testthat

Overview

Testing is essential for reliable R packages. This skill covers testthat 3rd edition, the standard testing framework for R packages, including test structure, expectations, fixtures, and advanced patterns.

Setup

Initial Setup

# Setup testthat 3rd edition:
usethis::use_testthat(3)

This creates:

tests/
├── testthat/
│   └── (test files will go here)
└── testthat.R

And adds to DESCRIPTION:

Suggests:
  testthat (>= 3.0.0)
Config/testthat/edition: 3

Key Differences: Edition 3 vs 2

Edition 3 changes:

• context() deprecated (use file names)
• More informative error messages
• Better snapshot testing
• Improved parallel test support
• Stricter comparison defaults

# OLD (Edition 2):
context("My feature tests")
expect_equal(x, y, tolerance = 1e-8)

# NEW (Edition 3):
# No context() - file name is context
expect_equal(x, y, tolerance = 1e-8)  # Same, but better errors

Test File Structure

Basic Structure

# tests/testthat/test-my-feature.R

test_that("basic functionality works", {
  result <- my_function(1:10)
  expect_equal(result, (1:10) * 2)
  expect_length(result, 10)
})

test_that("handles edge cases", {
  expect_equal(my_function(numeric(0)), numeric(0))
  expect_error(my_function(NULL), class = "error")
})

test_that("parameter validation works", {
  expect_error(my_function("not numeric"), "must be numeric")
  expect_warning(my_function(c(1, NA)), "NA values detected")
})

File Naming Convention

Mirror your R/ file structure:

R/
├── data-processing.R
├── visualization.R
└── utils.R

tests/testthat/
├── test-data-processing.R
├── test-visualization.R
└── test-utils.R

Rules:

• Files MUST start with test-
• Use descriptive names matching R/ files
• One test file per R source file (usually)
• Or organize by feature/functionality

Test Organization Within Files

# tests/testthat/test-statistics.R

# Group related tests
test_that("mean calculation is correct", {
  expect_equal(my_mean(1:10), 5.5)
  expect_equal(my_mean(c(1, 2, 3)), 2)
})

test_that("mean handles NA values", {
  expect_equal(my_mean(c(1, NA, 3), na.rm = TRUE), 2)
  expect_true(is.na(my_mean(c(1, NA, 3), na.rm = FALSE)))
})

test_that("mean validates input", {
  expect_error(my_mean("not numeric"))
  expect_error(my_mean(list(1, 2, 3)))
})

# Separate feature
test_that("median calculation is correct", {
  expect_equal(my_median(1:10), 5.5)
  expect_equal(my_median(1:11), 6)
})

Core Expectations

expect_equal()

Tests near equality (with tolerance for numerics).

test_that("numeric equality works", {
  # Exact equality:
  expect_equal(1 + 1, 2)

  # With tolerance (default 1.5e-8):
  expect_equal(sqrt(2)^2, 2)

  # Custom tolerance:
  expect_equal(1.00001, 1, tolerance = 1e-4)

  # Vectors:
  expect_equal(1:5, c(1, 2, 3, 4, 5))

  # Data frames:
  expect_equal(
    data.frame(x = 1:3, y = 4:6),
    data.frame(x = 1:3, y = 4:6)
  )

  # Ignore attributes:
  expect_equal(
    c(a = 1, b = 2),
    c(1, 2),
    ignore_attr = TRUE
  )
})

expect_identical()

Tests exact identity (no tolerance).

test_that("exact identity works", {
  # Integers:
  expect_identical(1L, 1L)

  # But not with doubles:
  expect_failure(expect_identical(1, 1L))  # Different types

  # Attributes matter:
  expect_failure(
    expect_identical(
      c(a = 1, b = 2),
      c(1, 2)
    )
  )

  # Use for reference comparisons:
  x <- 1:10
  y <- x
  expect_identical(x, y)

  # Type checking:
  expect_identical(class(x), "integer")
})

expect_error()

Tests that code throws an error.

test_that("errors are thrown correctly", {
  # Basic error:
  expect_error(stop("oops"))

  # Error with specific message (regex):
  expect_error(
    stop("value must be numeric"),
    "must be numeric"
  )

  # Error with specific class (PREFERRED):
  expect_error(
    my_function(invalid_input),
    class = "invalid_input_error"
  )

  # Both class and message:
  expect_error(
    my_function(NULL),
    "cannot be NULL",
    class = "null_input_error"
  )
})

Best practice: Use custom error classes and test them:

# In your package:
validate_input <- function(x) {
  if (!is.numeric(x)) {
    rlang::abort(
      "Input must be numeric",
      class = "invalid_input_error"
    )
  }
}

# In tests:
test_that("validation errors have correct class", {
  expect_error(
    validate_input("text"),
    class = "invalid_input_error"
  )
})

expect_warning()

Tests that code produces warnings.

test_that("warnings are issued correctly", {
  # Basic warning:
  expect_warning(warning("careful!"))

  # Warning with message:
  expect_warning(
    my_function(c(1, NA)),
    "NA values detected"
  )

  # Warning with class:
  expect_warning(
    my_function(x),
    class = "deprecated_argument"
  )
})

expect_message()

Tests that code produces messages.

test_that("messages are printed correctly", {
  expect_message(message("Processing..."))

  expect_message(
    my_function(verbose = TRUE),
    "Starting computation"
  )

  # Multiple messages:
  expect_message(
    expect_message(
      my_verbose_function(),
      "Step 1"
    ),
    "Step 2"
  )
})

expect_no_error() / expect_no_warning() / expect_no_message()

Tests that code runs without conditions.

test_that("clean execution", {
  # No errors:
  expect_no_error(my_function(valid_input))

  # No warnings:
  expect_no_warning(my_function(good_data))

  # No messages:
  expect_no_message(my_function(verbose = FALSE))
})

Other Useful Expectations

test_that("various expectations work", {
  # Truth values:
  expect_true(2 + 2 == 4)
  expect_false(2 + 2 == 5)

  # NULL:
  expect_null(NULL)
  expect_null(my_function_returning_null())

  # Type checks:
  expect_type(1:10, "integer")
  expect_type(letters, "character")

  # Class checks:
  expect_s3_class(lm(y ~ x, data), "lm")
  expect_s4_class(object, "myS4class")

  # Length/dimensions:
  expect_length(1:10, 10)
  expect_length(list(a = 1, b = 2), 2)

  # Named:
  expect_named(c(a = 1, b = 2), c("a", "b"))

  # Vector type and length:
  expect_vector(1:10, ptype = integer(), size = 10)

  # Matching:
  expect_match("hello world", "hello")
  expect_match("abc123", "\\d+")  # Regex

  # Set operations:
  expect_setequal(c(1, 2, 3), c(3, 2, 1))  # Order doesn't matter
  expect_contains(1:10, c(5, 7, 9))  # Subset

  # Invisible return:
  expect_invisible(invisible(42))

  # Output:
  expect_output(print("hello"), "hello")
})

Snapshot Testing

Test output that's hard to describe with expectations.

expect_snapshot()

Captures printed output, messages, warnings, and errors.

test_that("function output is correct", {
  expect_snapshot({
    my_complex_function()
  })
})

First run creates tests/testthat/_snaps/my-test.md:

# function output is correct

    Code
      my_complex_function()
    Output
      Processing data...
      Results:
        Mean: 5.5
        SD: 2.87

Subsequent runs compare against snapshot. Update with:

testthat::snapshot_review()
# Or:
testthat::snapshot_accept()

Snapshot Variants

test_that("snapshots capture different outputs", {
  # Just messages:
  expect_snapshot(
    message("Hello"),
    cnd_class = TRUE  # Include condition class
  )

  # Errors (with class):
  expect_snapshot(
    error = TRUE,
    my_function(invalid)
  )

  # Transforming output:
  expect_snapshot(
    my_function(),
    transform = scrub_randomness
  )

  # Multiple variants:
  expect_snapshot({
    cat("Output 1\n")
    message("Message 1")
    cat("Output 2\n")
  })
})

expect_snapshot_output()

Specifically for printed output (deprecated, use expect_snapshot()).

test_that("print methods work", {
  expect_snapshot_output(print(my_object))
})

expect_snapshot_value()

Captures R object structure.

test_that("complex return values are correct", {
  result <- my_complex_function()

  expect_snapshot_value(
    result,
    style = "json2"  # or "serialize", "deparse"
  )
})

When to Use Snapshots

Good for:

• Complex printed output (print methods, summaries)
• Error messages (ensures consistent UX)
• Multi-line formatted output
• Plots (as text representation)

Avoid for:

• Simple values (use expect_equal())
• When snapshots are hard to review
• When output changes frequently

Test Helpers and Fixtures

Helper Files

Files starting with helper- run before tests and make utilities available:

# tests/testthat/helper-data.R

# Create test data used across multiple test files:
make_test_data <- function(n = 100) {
  data.frame(
    id = seq_len(n),
    value = rnorm(n),
    category = sample(LETTERS[1:3], n, replace = TRUE)
  )
}

# Create test fixtures:
sample_data <- make_test_data()

# Utility assertions:
expect_valid_output <- function(x) {
  expect_s3_class(x, "data.frame")
  expect_true(nrow(x) > 0)
  expect_named(x, c("id", "result"))
}

Use in tests:

# tests/testthat/test-analysis.R
test_that("analysis works with test data", {
  result <- analyze(sample_data)
  expect_valid_output(result)
})

Setup Files

setup.R runs once before all tests:

# tests/testthat/setup.R