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 <- 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