tables-and-dynamic-specs

Table specs and dynamically generated specs

Ginkgo gives you a DSL for table-driven specs plus idioms for generating specs from loops and data. All of it is syntactic sugar that runs during the tree-construction phase (ginkgo:overview) — the gotchas all follow from that.

Perfer DescrtibeTable/DescribeTableSubtree with shared configuration over multiple repeated Its.

Docs: https://onsi.github.io/ginkgo/#table-specs.

DescribeTable + Entry

DescribeTable(desc, specFunc, ...Entry) generates one container holding one It per Entry. Entry(desc, params...) — its params are passed to specFunc at run time and must match specFunc's signature (you get a clear runtime message if they don't).

DescribeTable("Extracting the author's first and last name",
	func(author string, isValid bool, firstName, lastName string) {
		book := &books.Book{Title: "My Book", Author: author, Pages: 10}
		Expect(book.IsValid()).To(Equal(isValid))
		Expect(book.AuthorFirstName()).To(Equal(firstName))
		Expect(book.AuthorLastName()).To(Equal(lastName))
	},
	Entry("both names", "Victor Hugo", true, "Victor", "Hugo"),
	Entry("one name", "Hugo", true, "", "Hugo"),
	Entry("no name", "", false, "", ""),
)

A DescribeTable is just a container, so nest it inside Describe/Context and surround it with BeforeEach — setup runs fresh before each entry's spec.

THE gotcha: Entry params are evaluated at construction time

Entry(...) arguments are evaluated when the tree is built — before any BeforeEach has run. So an Entry cannot read a variable initialized in BeforeEach; it will see the zero value (a nil map/pointer).

var shelf map[string]*books.Book
BeforeEach(func() { shelf = loadShelf() }) // runs at RUN time

// WRONG — shelf is nil when Entry is evaluated at construction time
DescribeTable("category", func(b *books.Book, c books.Category) { ... },
	Entry("novel", shelf["Les Miserables"], books.CategoryNovel), // nil pointer!
)

// RIGHT — pass a key, dereference shelf inside the spec closure (run time)
DescribeTable("category", func(key string, c books.Category) {
	Expect(shelf[key].Category()).To(Equal(c))
},
	Entry("novel", "Les Miserables", books.CategoryNovel),
)

The four ways to describe an Entry

The description closure must return string and accept the same params as specFunc. Per-entry, the first arg may itself be a closure or EntryDescription (overrides the table default):

DescribeTable("addition", func(a, b, c int) { Expect(a + b).To(Equal(c)) },
	EntryDescription("%d + %d = %d"),                              // table default
	Entry(nil, 1, 2, 3),                                          // "1 + 2 = 3"
	Entry("zeros", 0, 0, 0),                                      // explicit
	Entry(EntryDescription("%[3]d = %[1]d + %[2]d"), 10, 100, 110), // per-entry override
	Entry(func(a, b, c int) string { return fmt.Sprintf("%d = %d", a+b, c) }, 4, 3, 7),
)

Decorating entries

Entry and DescribeTable accept every decorator (→ ginkgo:decorators): Entry("flaky case", FlakeAttempts(3), ...), Entry(..., Label("slow")). Focus/pending shortcuts: FEntry / PEntry / XEntry (and FDescribeTable / PDescribeTable). PEntry needs no params; focus/pending obey the same precedence as everywhere else (→ ginkgo:filtering).

DescribeTableSubtree — many Its per row

When you want a whole subtree (multiple Its, their own setup) per entry, use DescribeTableSubtree. Its body runs at construction time, once per entry, inside a fresh container — you must place Its inside it or no specs are generated:

DescribeTableSubtree("handling requests",
	func(url string, code int, message string) {
		var resp *http.Response
		BeforeEach(func() {
			var err error
			resp, err = http.Get(url)
			Expect(err).NotTo(HaveOccurred())
			DeferCleanup(resp.Body.Close)
		})
		It("returns the status code", func() { Expect(resp.StatusCode).To(Equal(code)) })
		It("returns the message", func() {
			body, _ := io.ReadAll(resp.Body)
			Expect(string(body)).To(Equal(message))
		})
	},
	Entry("default", "example.com/response", http.StatusOK, "hello world"),
	Entry("missing", "example.com/missing", http.StatusNotFound, "wat?"),
)

Patterns

• Struct-per-row for many params — inscrutable positional entries like Entry(nil, 12, 1.2, 8.5, 11, 2783) are unreadable. Define a type and pass it: Entry(nil, BookFormatting{FontSize: 12, LineHeight: 1.2, ...}, 2783).
• Reusable []TableEntry — share one entry set across tables: var InvalidBooks = []TableEntry{ Entry("empty", &books.Book{}), ... }, then DescribeTable("storing errors", storeFn, InvalidBooks) and DescribeTable("reading errors", readFn, InvalidBooks). Or feed the slice to DescribeTableSubtree to attach multiple specs per entry.

Loading fixture data: do it in TestXxx, NOT BeforeSuite

If the spec structure depends on external data, that data must be available during tree construction. BeforeSuite runs in the run phase — too late; a loop reading a BeforeSuite-populated slice generates zero specs. Load it in the TestXxx bootstrap function before RunSpecs:

var fixtureBooks []*books.Book

func TestBooks(t *testing.T) {
	RegisterFailHandler(Fail)
	g := NewGomegaWithT(t) // wrap t to assert before RunSpecs
	fixtureBooks = LoadFixturesFrom("./fixtures/books.json")
	g.Expect(fixtureBooks).NotTo(BeEmpty())
	RunSpecs(t, "Books Suite")
}

var _ = Describe("fixtures", func() {
	for _, book := range fixtureBooks { // populated before construction — works
		book := book
		It("stores "+book.Title, func() { Expect(library.Store(book)).To(Succeed()) })
	}
})

This works because TestBooks runs before tree construction, so fixtureBooks is populated when the loop runs, and because the function passed to Describe is not invoked until tree construction time. Don't load it in BeforeSuite — it's too late, and the loop generates no specs.

Docs: https://onsi.github.io/ginkgo/#dynamically-generating-specs.

Shared behaviors

To reuse identical Its across Contexts that differ only in setup, put the Its in a closure and call it inside each Context body (it runs at construction time, adding those specs to each context). Because the closure is defined in the same scope, it closes over the shared variable that each Context's BeforeEach configures:

AssertFailedBehavior := func() {
	It("can't be stored", func() { Expect(library.IsStorable(book)).To(BeFalse()) })
	It("fails to store", func() { Expect(library.Store(book)).To(MatchError(books.ErrStoringBook)) })
}
Context("when the book has no title", func() {
	BeforeEach(func() { book = &books.Book{Author: "Victor Hugo", Pages: 2783} })
	AssertFailedBehavior()
})
Context("when the book is nil", func() {
	BeforeEach(func() { book = nil })
	AssertFailedBehavior()
})

Docs: https://onsi.github.io/ginkgo/#shared-behaviors, https://onsi.github.io/ginkgo/#table-specs-patterns.