r-testthat

Files (6)

• SKILL.md
• advanced.md · references/
• bdd.md · references/
• fixtures.md · references/
• mocking.md · references/
• snapshots.md · references/

SKILL.md (first lines)

# Testing R Packages with testthat

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

## Initial Setup

Initialize testing with testthat 3rd edition:

```r
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

```r
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()`:

```r
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](references/bdd.md) for comprehensive BDD patterns, nested specifications, and test-first workflows.

## Running Tests

Three scales of testing:

**Micro** (interactive development):
```r
devtools::load_all()
expect_equal(foofy(...), expected)
```

**Mezzo** (single file):
```r
testthat::test_file("tests/testthat/test-foofy.R")
# RStudio: Ctrl/Cmd + Shift + T
```

**Macro** (full suite):
```r
devtools::test()    # Ctrl/Cmd + Shift + T
devtools::check()   # Ctrl/Cmd + Shift + E
```

## Core Expectations

### Equality

```r
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

```r
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

```r
expect_match("Testing is fun!", "Testing")

From posit-dev/skills, MIT.

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