golang-testing

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 requirement

Step-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 pass

Table-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/op

Benchmark 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=30s

Fuzz 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.