18 — Testing

A func TestX(t *testing.T) and go test are where most Go programmers stop reading the testing ↗ package. This chapter is the rest of it, measured on Go 1.25: subtests that turn a table of cases into named, individually runnable tests; Example functions whose // Output: comment is checked on every run; t.Parallel, t.Cleanup, t.TempDir, t.Setenv and t.Context, which share one test process safely; fuzzing, whose seed corpus is an ordinary deterministic test; and the flags that choose, cache, shuffle and measure — -run, -skip, -count, -cover, -shuffle, -short, -failfast.

Every example is a bash driver that writes a throwaway module, runs go test, and prints the transcript with the durations stripped, so what the pages show is what go test printed on both CI machines. The reader should know a TestX and a t.Errorf; The race detector and synctest.Wait instead of a sleep cover -race and testing/synctest and are not repeated here. Benchmarks are in Performance.

Lesson	The one thing
A table case is a subtest with t.Run	go test -v names each case TestParseQuantity/empty, -run selects one, a failing case does not stop its siblings, and t.Fatal ends only its own subtest
An Example test runs its // Output:	an Example with an // Output: comment is compiled and run, a wrong comment fails with got:/want:, and one without a comment is compiled but never run
Parallel subtests wait, Cleanup runs last	t.Parallel subtests pause until the parent returns, t.Cleanup runs after the subtests in reverse order, t.Context is cancelled first, and t.Setenv in a parallel test panics
Fuzzing finds the input you did not write	without -fuzz a fuzz target runs its seed corpus and is deterministic; a corpus file written by hand pins a crashing input; -fuzz finds one you did not think of
Coverage and the flags that choose tests	-cover and go tool cover -func are exact for fixed code, -run/-skip match per /-separated level, (cached) appears only with a package argument, and -shuffle=1 is repeatable

Companion	What it holds
Testing: the errors go test stops on	a wrong TestX signature, an unexported name from an external test package, an unsupported fuzz argument, [no test files], a panic that ends the binary, and Fail in goroutine after TestX has completed
Testing: what go vet catches	the tests, printf, testinggoroutine, waitgroup and copylocks analyzers on test code, which of them go test runs on its own, and what no tool catches
Testing: resources	the package documentation, the Go blog posts on subtests, examples, fuzzing and coverage, the release notes, and the books' chapters

Planned

• TestMain — one func TestMain(m *testing.M) per package for set-up that every test shares, m.Run(), and why a TestMain that forgets to call os.Exit with the result used to hide failures.
• httptest and iotest — the standard library's test doubles for HTTP servers and misbehaving readers.
• Benchmarks in this chapter's terms — b.Loop, sub-benchmarks with b.Run, and -benchmem; the measurements live in Performance.
• go test -json and test2json — machine-readable events for a CI dashboard.