Testing Best Practices

Write reliable, maintainable tests that give confidence in code correctness.

When to Use

• Writing new tests for features or bug fixes
• Improving test coverage in under-tested areas
• Refactoring flaky or brittle tests
• Reviewing test quality during code review
• Adding regression tests for discovered bugs

When NOT to Use

• Running tests without modifications
• Setting up test infrastructure (CI/CD, framework installation)
• Performance/load testing (use dedicated load testing tools)

Test Pyramid

Structure your test suite using the test pyramid model:

        /\
       /  \         Unit Tests (Fast, many)
      /    \
     /------\
    /        \      Integration Tests (Medium, fewer)
   /----------\
  /            \    E2E Tests (Slow, few)
 /--------------\

Unit Tests (Base Layer)

• Purpose: Test individual functions/methods in isolation
• Speed: Milliseconds
• Quantity: Most tests should be unit tests
• Isolation: No external dependencies (network, database, filesystem)

// Good: Pure unit test, no dependencies
func TestCalculateTotal(t *testing.T) {
    items := []Item{
        {Price: 10.00, Qty: 2},
        {Price: 5.00, Qty: 1},
    }
    
    got := CalculateTotal(items)
    want := 25.00
    
    if got != want {
        t.Errorf("CalculateTotal() = %v, want %v", got, want)
    }
}

Integration Tests (Middle Layer)

• Purpose: Test component interactions and external integrations
• Speed: Seconds
• Quantity: Moderate
• Isolation: May use test databases, mock external APIs

// Integration test with database
func TestUserRepository_Create(t *testing.T) {
    db := setupTestDB(t)
    defer cleanupTestDB(t, db)
    
    repo := NewUserRepository(db)
    user := &User{Name: "Alice", Email: "[email protected]"}
    
    err := repo.Create(user)
    if err != nil {
        t.Fatalf("Create() error = %v", err)
    }
    
    if user.ID == 0 {
        t.Error("expected user ID to be set")
    }
}

End-to-End Tests (Top Layer)

• Purpose: Test complete user workflows through the full stack
• Speed: Seconds to minutes
• Quantity: Few, focused on critical paths
• Isolation: Full environment required

// E2E test hitting real HTTP endpoint
func TestLoginEndpoint(t *testing.T) {
    server := startTestServer(t)
    defer server.Close()
    
    resp, err := http.Post(
        server.URL+"/api/login",
        "application/json",
        strings.NewReader(`{"email":"[email protected]","password":"secret"}`),
    )
    if err != nil {
        t.Fatalf("request failed: %v", err)
    }
    defer resp.Body.Close()
    
    if resp.StatusCode != http.StatusOK {
        t.Errorf("status = %d, want %d", resp.StatusCode, http.StatusOK)
    }
}

Arrange-Act-Assert Pattern

Structure every test with three clear phases:

func TestDiscountCalculation(t *testing.T) {
    // ARRANGE: Set up test data and dependencies
    product := &Product{Price: 100.00}
    discount := 0.20 // 20% off
    
    // ACT: Execute the function under test
    result := CalculateDiscount(product.Price, discount)
    
    // ASSERT: Verify the expected outcome
    want := 80.00
    if result != want {
        t.Errorf("CalculateDiscount() = %v, want %v", result, want)
    }
}

Why AAA?

• Readability: Clear separation of concerns
• Maintainability: Easy to identify which phase breaks
• Documentation: Test intent is obvious

AAA Variants

// Given-When-Then (BDD style)
func TestLoginValidation(t *testing.T) {
    // Given: valid credentials
    credentials := Credentials{Email: "[email protected]", Password: "valid123"}
    
    // When: user attempts login
    result, err := AuthService.Login(credentials)
    
    // Then: login succeeds
    if err != nil {
        t.Errorf("unexpected error: %v", err)
    }
    if result.Token == "" {
        t.Error("expected token to be set")
    }
}

// Arrange-Act-Assert-Cleanup (for resources)
func TestFileProcessing(t *testing.T) {
    // Arrange
    tmpFile := createTempFile(t, "test data")
    defer os.Remove(tmpFile)
    
    // Act
    result, err := ProcessFile(tmpFile)
    
    // Assert
    if err != nil {
        t.Fatalf("ProcessFile() error = %v", err)
    }
    if result.Processed != true {
        t.Error("expected file to be marked as processed")
    }
}

Mocking and Stubbing

When to Mock

• External services (APIs, databases, file systems)
• Time-dependent code
• Random/non-deterministic values
• Slow operations

Interface-Based Mocking (Go)

Define interfaces for dependencies:

// Production code depends on interface, not concrete type
type UserStore interface {
    GetByID(id int64) (*User, error)
    Save(user *User) error
}

// Mock implementation
type MockUserStore struct {
    Users  map[int64]*User
    SaveFn func(user *User) error
}

func (m *MockUserStore) GetByID(id int64) (*User, error) {
    user, ok := m.Users[id]
    if !ok {
        return nil, ErrNotFound
    }
    return user, nil
}

func (m *MockUserStore) Save(user *User) error {
    if m.SaveFn != nil {
        return m.SaveFn(user)
    }
    m.Users[user.ID] = user
    return nil
}

Using Mocks in Tests

func TestGetUserProfile(t *testing.T) {
    // Arrange
    mockStore := &MockUserStore{
        Users: map[int64]*User{
            1: {ID: 1, Name: "Alice", Email: "[email protected]"},
        },
    }
    service := NewUserService(mockStore)
    
    // Act
    profile, err := service.GetProfile(1)
    
    // Assert
    if err != nil {
        t.Fatalf("GetProfile() error = %v", err)
    }
    if profile.Name != "Alice" {
        t.Errorf("Name = %q, want %q", profile.Name, "Alice")
    }
}

func TestSaveUser_RejectsInvalidEmail(t *testing.T) {
    // Arrange
    mockStore := &MockUserStore{
        Users: make(map[int64]*User),
    }
    service := NewUserService(mockStore)
    user := &User{Name: "Bob", Email: "invalid"}
    
    // Act
    err := service.Save(user)
    
    // Assert
    if err == nil {
        t.Error("expected error for invalid email, got nil")
    }
}

Table-Driven Tests

For testing multiple cases efficiently:

func TestValidateEmail(t *testing.T) {
    tests := []struct {
        name    string
        email   string
        wantErr bool
    }{
        {name: "valid email", email: "[email protected]", wantErr: false},
        {name: "missing @", email: "aliceexample.com", wantErr: true},
        {name: "missing domain", email: "alice@", wantErr: true},
        {name: "empty string", email: "", wantErr: true},
    }
    
    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            err := ValidateEmail(tt.email)
            if (err != nil) != tt.wantErr {
                t.Errorf("ValidateEmail(%q) error = %v, wantErr %v",
                    tt.email, err, tt.wantErr)
            }
        })
    }
}

Coverage Guidelines

Coverage Targets

What to Test

Always test:

• Happy paths (expected inputs)
• Error cases (invalid inputs, edge cases)
• Boundary conditions (empty, nil, max values)
• State transitions

Don't obsess over:

• Getters/setters
• Framework boilerplate
• Third-party library internals

Coverage in Go

# Generate coverage report
go test -coverprofile=coverage.out ./...

# View coverage in terminal
go tool cover -func=coverage.out

# Generate HTML report for detailed view
go tool cover -html=coverage.out -o coverage.html

Reading Coverage Output

ok  github.com/example/pkg  0.5s  coverage: 85.2% of statements

Focus on increasing coverage for files with lowest percentages:

# Show coverage per function
go tool cover -func=coverage.out | sort -k3 -n

Test Organization

File Structure

src/
├── user.go              # Production code
├── user_test.go         # Unit tests (same package)
├── user_integration_test.go  # Integration tests
└── testdata/
    └── fixtures/        # Test data files

Naming Conventions

// Function under test: CalculateDiscount

// Standard test (success case)
func TestCalculateDiscount(t *testing.T) { ... }

// Failure/edge case
func TestCalculateDiscount_ZeroDiscount(t *testing.T) { ... }
func TestCalculateDiscount_NegativeAmount(t *testing.T) { ... }

// Benchmark
func BenchmarkCalculateDiscount(b *testing.B) { ... }

// Example (documentation)
func ExampleCalculateDiscount() { ... }

Edge Cases to Cover

Nil/Empty Inputs

func TestProcessItems_EmptySlice(t *testing.T) {
    result := ProcessItems([]Item{})
    if len(result) != 0 {
        t.Error("expected empty result for empty input")
    }
}

func TestProcessItems_NilSlice(t *testing.T) {
    result := ProcessItems(nil)
    if result != nil {
        t.Error("expected nil result for nil input")
    }
}

Boundary Values

func TestAgeValidation(t *testing.T) {
    tests := []struct {
        age     int
        wantErr bool
    }{
        {age: 0, wantErr: true},    // Minimum boundary
        {age: 1, wantErr: false},   // Just above minimum
        {age: 17, wantErr: false},  // Below adult age
        {age: 18, wantErr: false},  // Adult threshold
        {age: 150, wantErr: false}, // Reasonable maximum
        {age: 151, wantErr: true},  // Above reasonable max
    }
    // ... test loop
}

Concurrent Access

func TestConcurrentAccess(t *testing.T) {
    cache := NewCache()
    var wg sync.WaitGroup
    
    for i := 0; i < 100; i++ {
        wg.Add(1)
        go func(n int) {
            defer wg.Done()
            cache.Set(fmt.Sprintf("key%d", n), n)
            _ = cache.Get(fmt.Sprintf("key%d", n))
        }(i)
    }
    
    wg.Wait()
    // Cache should not panic or corrupt data
}

Common Anti-Patterns

Avoid

• Tests that depend on execution order — each test should be independent
• Mocking everything — prefer real implementations when practical
• Testing implementation details — test behavior, not internals
• Sleep-based synchronization — use channels or callbacks
• Ignored errors — always check error returns

Good vs Bad Test

// BAD: Tests implementation detail, brittle
func TestUserRepository_InternalCache(t *testing.T) {
    repo := NewUserRepository(db)
    repo.cache = map[int64]*User{}  // Testing internal state
    
    user := repo.GetUser(1)
    if _, exists := repo.cache[1]; !exists {
        t.Error("expected cache to be populated")
    }
}

// GOOD: Tests observable behavior
func TestUserRepository_GetUser_CachesResult(t *testing.T) {
    db := &MockDB{QueryCount: 0}
    repo := NewUserRepository(db)
    
    // First call hits database
    _, _ = repo.GetUser(1)
    if db.QueryCount != 1 {
        t.Errorf("expected 1 query, got %d", db.QueryCount)
    }
    
    // Second call uses cache (no additional query)
    _, _ = repo.GetUser(1)
    if db.QueryCount != 1 {
        t.Errorf("expected still 1 query, got %d", db.QueryCount)
    }
}

Quick Reference

Test Command

go test ./...                    # Run all tests
go test -v ./...                 # Verbose output
go test -run TestFunctionName    # Run specific test
go test -count=3 ./...           # Run 3 times (find flaky tests)
go test -race ./...              # Detect race conditions

Test Flags

go test -cover                   # Show coverage percentage
go test -coverprofile=out.out    # Save coverage data
go test -bench=.                 # Run benchmarks
go test -short                   # Skip long-running tests
go test -timeout 30s             # Fail if test exceeds timeout

Best Practices

1. One assertion per concept — each test should verify one behavior
2. Descriptive names — test names should explain what's being tested
3. Independent tests — no test should depend on another test's state
4. Fast feedback — keep unit tests under 100ms each
5. Test behavior, not implementation — verify outcomes, not methods called
6. Keep tests readable — a test is documentation
7. Refactor tests — extract helpers for repeated setup
8. Delete dead tests — if code is removed, remove its tests too