SKILL.md
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 requirementStep-by-Step TDD in 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 passTable-Driven Tests
The standard pattern for Go tests. Enables comprehensive coverage with minimal code.
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
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
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
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
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
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/.
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
// 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
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/opBenchmark with Different Sizes
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
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
func FuzzParseJSON(f *testing.F) {
// Add seed corpus
f.Add(`{"name": "test"}`)
f.Add(`{"count": 123}`)
f.Add(`[]`)
f.Add(`""`)
f.Fuzz(func(t *testing.T, input string) {
var result map[string]interface{}
err := json.Unmarshal([]byte(input), &result)
if err != nil {
// Invalid JSON is expected for random input
return
}
// If parsing succeeded, re-encoding should work
_, err = json.Marshal(result)
if err != nil {
t.Errorf("Marshal failed after successful Unmarshal: %v", err)
}
})
}
// Run: go test -fuzz=FuzzParseJSON -fuzztime=30sFuzz Test with Multiple Inputs
func FuzzCompare(f *testing.F) {
f.Add("hello", "world")
f.Add("", "")
f.Add("abc", "abc")
f.Fuzz(func(t *testing.T, a, b string) {
result := Compare(a, b)
// Property: Compare(a, a) should always equal 0
if a == b && result != 0 {
t.Errorf("Compare(%q, %q) = %d; want 0", a, b, result)
}
// Property: Compare(a, b) and Compare(b, a) should have opposite signs
reverse := Compare(b, a)
if (result > 0 && reverse >= 0) || (result < 0 && reverse <= 0) {
if result != 0 || reverse != 0 {
t.Errorf("Compare(%q, %q) = %d, Compare(%q, %q) = %d; inconsistent",
a, b, result, b, a, reverse)
}
}
})
}Test Coverage
Running Coverage
# Basic coverage
go test -cover ./...
# Generate coverage profile
go test -coverprofile=coverage.out ./...
# View coverage in browser
go tool cover -html=coverage.out
# View coverage by function
go tool cover -func=coverage.out
# Coverage with race detection
go test -race -coverprofile=coverage.out ./...Coverage Targets
Code Type
Target
Critical business logic
100%
Public APIs
90%+
General code
80%+
Generated code
Exclude
Excluding Generated Code from Coverage
//go:generate mockgen -source=interface.go -destination=mock_interface.go
// In coverage profile, exclude with build tags:
// go test -cover -tags=!generate ./...HTTP Handler Testing
func TestHealthHandler(t *testing.T) {
// Create request
req := httptest.NewRequest(http.MethodGet, "/health", nil)
w := httptest.NewRecorder()
// Call handler
HealthHandler(w, req)
// Check response
resp := w.Result()
defer resp.Body.Close()
if resp.StatusCode != http.StatusOK {
t.Errorf("got status %d; want %d", resp.StatusCode, http.StatusOK)
}
body, _ := io.ReadAll(resp.Body)
if string(body) != "OK" {
t.Errorf("got body %q; want %q", body, "OK")
}
}
func TestAPIHandler(t *testing.T) {
tests := []struct {
name string
method string
path string
body string
wantStatus int
wantBody string
}{
{
name: "get user",
method: http.MethodGet,
path: "/users/123",
wantStatus: http.StatusOK,
wantBody: `{"id":"123","name":"Alice"}`,
},
{
name: "not found",
method: http.MethodGet,
path: "/users/999",
wantStatus: http.StatusNotFound,
},
{
name: "create user",
method: http.MethodPost,
path: "/users",
body: `{"name":"Bob"}`,
wantStatus: http.StatusCreated,
},
}
handler := NewAPIHandler()
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
var body io.Reader
if tt.body != "" {
body = strings.NewReader(tt.body)
}
req := httptest.NewRequest(tt.method, tt.path, body)
req.Header.Set("Content-Type", "application/json")
w := httptest.NewRecorder()
handler.ServeHTTP(w, req)
if w.Code != tt.wantStatus {
t.Errorf("got status %d; want %d", w.Code, tt.wantStatus)
}
if tt.wantBody != "" && w.Body.String() != tt.wantBody {
t.Errorf("got body %q; want %q", w.Body.String(), tt.wantBody)
}
})
}
}Testing Commands
# Run all tests
go test ./...
# Run tests with verbose output
go test -v ./...
# Run specific test
go test -run TestAdd ./...
# Run tests matching pattern
go test -run "TestUser/Create" ./...
# Run tests with race detector
go test -race ./...
# Run tests with coverage
go test -cover -coverprofile=coverage.out ./...
# Run short tests only
go test -short ./...
# Run tests with timeout
go test -timeout 30s ./...
# Run benchmarks
go test -bench=. -benchmem ./...
# Run fuzzing
go test -fuzz=FuzzParse -fuzztime=30s ./...
# Count test runs (for flaky test detection)
go test -count=10 ./...Best Practices
DO:
- Write tests FIRST (TDD)
- Use table-driven tests for comprehensive coverage
- Test behavior, not implementation
- Use
t.Helper()in helper functions
- Use
t.Parallel()for independent tests
- Clean up resources with
t.Cleanup()
- Use meaningful test names that describe the scenario
DON'T:
- Test private functions directly (test through public API)
- Use
time.Sleep()in tests (use channels or conditions)
- Ignore flaky tests (fix or remove them)
- Mock everything (prefer integration tests when possible)
- Skip error path testing
Integration with CI/CD
# GitHub Actions example
test:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- uses: actions/setup-go@v5
with:
go-version: '1.22'
- name: Run tests
run: go test -race -coverprofile=coverage.out ./...
- name: Check coverage
run: |
go tool cover -func=coverage.out | grep total | awk '{print $3}' | \
awk -F'%' '{if ($1 < 80) exit 1}'Remember: Tests are documentation. They show how your code is meant to be used. Write them clearly and keep them up to date.