Go Test Quality

Tests are production code. They run in CI on every commit, they document behavior, and they're the first thing you read when a function breaks at 3am. Write them with the same care you'd give to code that handles money.

Detailed reference material, loaded on demand:

• references/helpers-and-fixtures.md — test helpers, factory functions with options, t.Cleanup, golden files, mock implementations.
• references/integration-testing.md — httptest recorder and server, testcontainers, build tags, TestMain, fuzz testing.

Read a reference file only when the summary below is not enough.

1. Test Design Philosophy

Test behavior, not implementation

// ✅ Good — tests what the function DOES
func TestTransferFunds_InsufficientBalance(t *testing.T) {
    from := NewAccount("alice", 100)
    to := NewAccount("bob", 0)

    err := TransferFunds(from, to, 150)

    require.ErrorIs(t, err, ErrInsufficientFunds)
    assert.Equal(t, 100, from.Balance(), "sender balance should be unchanged")
    assert.Equal(t, 0, to.Balance(), "receiver balance should be unchanged")
}

// ❌ Bad — tests HOW the function does it
// asserts debit() was called before credit(), rollback() was called,
// internal mutex was locked — breaks on every refactor

One assertion per logical concept

Multiple assert calls are fine when they verify different facets of the SAME behavior (both accounts after a transfer). A test that checks creation AND update AND deletion is three tests pretending to be one.

Name tests like bug reports

When the test fails, the name alone should say what broke:

// ✅ Good — reads like a sentence
func TestOrderService_Cancel_RefundsPartiallyShippedItems(t *testing.T) { ... }
func TestParseConfig_ReturnsErrorOnMissingRequiredField(t *testing.T) { ... }

// ❌ Bad — says nothing useful
func TestCancel(t *testing.T) { ... }
func TestRateLimiter_Success(t *testing.T) { ... }

2. Subtests for Organized Scenarios

Use t.Run to group related scenarios under a parent test. Each subtest gets its own setup, its own failure, and its own name in CI output:

func TestUserService_Create(t *testing.T) {
    svc := setupUserService(t)

    t.Run("succeeds with valid input", func(t *testing.T) {
        user, err := svc.Create(ctx, CreateUserInput{Name: "Alice", Email: "[email protected]"})
        require.NoError(t, err)
        assert.NotEmpty(t, user.ID)
    })

    t.Run("rejects duplicate email", func(t *testing.T) {
        _, _ = svc.Create(ctx, CreateUserInput{Name: "Alice", Email: "[email protected]"})
        _, err := svc.Create(ctx, CreateUserInput{Name: "Bob", Email: "[email protected]"})
        require.ErrorIs(t, err, ErrDuplicateEmail)
    })
}

3. Test Helper Rules

1. Always call t.Helper() in test utilities so failures point to the caller, not the helper.
2. Factory functions with functional options for complex test objects — defaults with per-test overrides, never a 15-parameter constructor.
3. Prefer t.Cleanup over defer — it runs even after t.FailNow() and is scoped to the test, not the function.

Full implementations in references/helpers-and-fixtures.md.

4. Choosing the Test Type

5. Mocking Rules

• Interface-based hand-written mocks for small interfaces (≤3 methods): a struct with function fields plus recorded calls.
• Function injection for simple seams (now func() time.Time).
• Do NOT mock: value objects, pure functions, the standard library, or your own code in the same package. Test the real thing.
• If you mock everything, you're testing your mocks, not your code.

6. Parallelism and Coverage

func TestSlugify(t *testing.T) {
    t.Parallel() // safe: pure function, no shared state
    // ...
}

Do NOT use t.Parallel() when tests share mutable state, databases, files, or process-level state (os.Setenv).

go test -race -coverprofile=coverage.out ./...
go tool cover -func=coverage.out

Targets: business logic 80%+, critical paths (auth, payments) 95%+, handlers 70%+. Don't chase 100% on generated code and simple getters.

Anti-Patterns

• 🔴 Test with no assertions — always passes, proves nothing
• 🔴 time.Sleep for synchronization — use channels or polling
• 🔴 Test depends on execution order — each test must stand alone
• 🔴 Mocking everything — you end up testing your mocks, not your code
• 🟡 Test names like Test1, TestSuccess — name the scenario
• 🟡 Reaching into private fields — test through the public API
• 🟡 No edge cases: empty, nil, zero, max values, unicode
• 🟡 Giant shared setup — each test should set up only what it needs
• 🟢 Fuzz anything that takes untrusted input
• 🟢 Golden files for complex output comparisons

Verification Checklist

1. Every test has meaningful assertions (no empty test bodies)
2. Test names describe the scenario, not the method
3. t.Helper() called in every test utility function
4. t.Cleanup() used for resource teardown
5. t.Parallel() used where safe, avoided where not
6. Integration tests guarded with testing.Short() or build tags
7. Mocks are minimal — only mock external dependencies
8. Edge cases covered: empty, nil, zero, boundary values
9. go test -race ./... passes
10. Coverage is meaningful, not just high numbers