golang-testing

# Go Testing Patterns Comprehensive Go testing patterns for writing reliable, maintainable tests following TDD methodology. ## When to Activate - Writing new Go functions or methods - Adding test coverage to existing code - Creating benchmarks for performance-critical code - Implementing fuzz tests for input validation - Following TDD workflow in Go projects ## TDD Workflow for Go ### The RED-GREEN-REFACTOR Cycle ``` RED → Write a failing test first GREEN → Write minimal code to pass the test REFACTOR → Improve code while keeping tests green REPEAT → Continue with next requirement ``` ### Step-by-Step TDD in Go ```go // Step 1: Define the interface/signature // calculator.go package calculator func Add(a, b int) int { panic("not implemented") // Placeholder } // Step 2: Write failing test (RED) // calculator_test.go package calculator import "testing" func TestAdd(t *testing.T) { got := Add(2, 3) want := 5 if got != want { t.Errorf("Add(2, 3) = %d; want %d", got, want) } } // Step 3: Run test - verify FAIL // $ go test // --- FAIL: TestAdd (0.00s) // panic: not implemented // Step 4: Implement minimal code (GREEN) func Add(a, b int) int { return a + b } // Step 5: Run test - verify PASS // $ go test // PASS // Step 6: Refactor if needed, verify tests still pass ``` ## Table-Driven Tests The standard pattern for Go tests. Enables comprehensive coverage with minimal code. ```go func TestAdd(t *testing.T) { tests := []struct { name string a, b int expected int }{ {"positive numbers", 2, 3, 5}, {"negative numbers", -1, -2, -3}, {"zero values", 0, 0, 0}, {"mixed signs", -1, 1, 0}, {"large numbers", 1000000, 2000000, 3000000}, } for _, tt := range tests { t.Run(tt.name, func(t *testing.T) { got := Add(tt.a, tt.b) if got != tt.expected { t.Errorf("Add(%d, %d) = %d; want %d", tt.a, tt.b, got, tt.expected) } }) } } ``` ### Table-Driven Tests with Error Cases ```go func TestParseConfig(t *testing.T) { tests := []struct { name string input string want *Config wantErr bool }{ { name: "valid config", input: `{"host": "localhost", "port": 8080}`, want: &Config{Host: "localhost", Port: 8080}, }, { name: "invalid JSON", input: `{invalid}`, wantErr: true, }, { name: "empty input", input: "", wantErr: true, }, { name: "minimal config", input: `{}`, want: &Config{}, // Zero value config }, } for _, tt := range tests { t.Run(tt.name, func(t *testing.T) { got, err := ParseConfig(tt.input) if tt.wantErr { if err == nil { t.Error("expected error, got nil") } return } if err != nil { t.Fatalf("unexpected error: %v", err) } if !reflect.DeepEqual(got, tt.want) { t.Errorf("got %+v; want %+v", got, tt.want) } }) } } ``` ## Subtests and Sub-benchmarks ### Organizing Related Tests ```go func TestUser(t *testing.T) { // Setup shared by all subtests db := setupTestDB(t) t.Run("Create", func(t *testing.T) { user := &User{Name: "Alice"} err := db.CreateUser(user) if err != nil { t.Fatalf("CreateUser failed: %v", err) } if user.ID == "" { t.Error("expected user ID to be set") } }) t.Run("Get", func(t *testing.T) { user, err := db.GetUser("alice-id") if err != nil { t.Fatalf("GetUser failed: %v", err) } if user.Name != "Alice" { t.Errorf("got name %q; want %q", user.Name, "Alice") } }) t.Run("Update", func(t *testing.T) { // ... }) t.Run("Delete", func(t *testing.T) { // ... }) } ``` ### Parallel Subtests ```go func TestParallel(t *testing.T) { tests := []struct { name string input string }{ {"case1", "input1"}, {"case2", "input2"}, {"case3", "input3"}, } for _, tt := range tests { tt := tt // Capture range variable t.Run(tt.name, func(t *testing.T) { t.Parallel() // Run subtests in parallel result := Process(tt.input) // assertions... _ = result }) } } ``` ## Test Helpers ### Helper Functions ```go func setupTestDB(t *testing.T) *sql.DB { t.Helper() // Marks this as a helper function db, err := sql.Open("sqlite3", ":memory:") if err != nil { t.Fatalf("failed to open database: %v", err) } // Cleanup when test finishes t.Cleanup(func() { db.Close() }) // Run migrations if _, err := db.Exec(schema); err != nil { t.Fatalf("failed to create schema: %v", err) } return db } func assertNoError(t *testing.T, err error) { t.Helper() if err != nil { t.Fatalf("unexpected error: %v", err) } } func assertEqual[T comparable](t *testing.T, got, want T) { t.Helper() if got != want { t.Errorf("got %v; want %v", got, want) } } ``` ### Temporary Files and Directories ```go func TestFileProcessing(t *testing.T) { // Create temp directory - automatically cleaned up tmpDir := t.TempDir() // Create test file testFile := filepath.Join(tmpDir, "test.txt") err := os.WriteFile(testFile, []byte("test content"), 0644) if err != nil { t.Fatalf("failed to create test file: %v", err) } // Run test result, err := ProcessFile(testFile) if err != nil { t.Fatalf("ProcessFile failed: %v", err) } // Assert... _ = result } ``` ## Golden Files Testing against expected output files stored in `testdata/`. ```go var update = flag.Bool("update", false, "update golden files") func TestRender(t *testing.T) { tests := []struct { name string input Template }{ {"simple", Template{Name: "test"}}, {"complex", Template{Name: "test", Items: []string{"a", "b"}}}, } for _, tt := range tests { t.Run(tt.name, func(t *testing.T) { got := Render(tt.input) golden := filepath.Join("testdata", tt.name+".golden") if *update { // Update golden file: go test -update err := os.WriteFile(golden, got, 0644) if err != nil { t.Fatalf("failed to update golden file: %v", err) } } want, err := os.ReadFile(golden) if err != nil { t.Fatalf("failed to read golden file: %v", err) } if !bytes.Equal(got, want) { t.Errorf("output mismatch:\ngot:\n%s\nwant:\n%s", got, want) } }) } } ``` ## Mocking with Interfaces ### Interface-Based Mocking ```go // Define interface for dependencies type UserRepository interface { GetUser(id string) (*User, error) SaveUser(user *User) error } // Production implementation type PostgresUserRepository struct { db *sql.DB } func (r *PostgresUserRepository) GetUser(id string) (*User, error) { // Real database query } // Mock implementation for tests type MockUserRepository struct { GetUserFunc func(id string) (*User, error) SaveUserFunc func(user *User) error } func (m *MockUserRepository) GetUser(id string) (*User, error) { return m.GetUserFunc(id) } func (m *MockUserRepository) SaveUser(user *User) error { return m.SaveUserFunc(user) } // Test using mock func TestUserService(t *testing.T) { mock := &MockUserRepository{ GetUserFunc: func(id string) (*User, error) { if id == "123" { return &User{ID: "123", Name: "Alice"}, nil } return nil, ErrNotFound }, } service := NewUserService(mock) user, err := service.GetUserProfile("123") if err != nil { t.Fatalf("unexpected error: %v", err) } if user.Name != "Alice" { t.Errorf("got name %q; want %q", user.Name, "Alice") } } ``` ## Benchmarks ### Basic Benchmarks ```go func BenchmarkProcess(b *testing.B) { data := generateTestData(1000) b.ResetTimer() // Don't count setup time for i := 0; i < b.N; i++ { Process(data) } } // Run: go test -bench=BenchmarkProcess -benchmem // Output: BenchmarkProcess-8 10000 105234 ns/op 4096 B/op 10 allocs/op ``` ### Benchmark with Different Sizes ```go func BenchmarkSort(b *testing.B) { sizes := []int{100, 1000, 10000, 100000} for _, size := range sizes { b.Run(fmt.Sprintf("size=%d", size), func(b *testing.B) { data := generateRandomSlice(size) b.ResetTimer() for i := 0; i < b.N; i++ { // Make a copy to avoid sorting already sorted data tmp := make([]int, len(data)) copy(tmp, data) sort.Ints(tmp) } }) } } ``` ### Memory Allocation Benchmarks ```go func BenchmarkStringConcat(b *testing.B) { parts := []string{"hello", "world", "foo", "bar", "baz"} b.Run("plus", func(b *testing.B) { for i := 0; i < b.N; i++ { var s string for _, p := range parts { s += p } _ = s } }) b.Run("builder", func(b *testing.B) { for i := 0; i < b.N; i++ { var sb strings.Builder for _, p := range parts { sb.WriteString(p) } _ = sb.String() } }) b.Run("join", func(b *testing.B) { for i := 0; i < b.N; i++ { _ = strings.Join(parts, "") } }) } ``` ## Fuzzing (Go 1.18+) ### Basic Fuzz Test ```go func FuzzParseJSON(f *testing.F) { // Add seed corpus f.Add(`{"name": "test"}`)