Go Testing Code Review

---
name: go-testing-code-review
description: Reviews Go test code for proper table-driven tests, assertions, and coverage patterns. Use when reviewing *_test.go files.
---

# Go Testing Code Review

## Quick Reference

| Issue Type | Reference |
|------------|-----------|
| Test structure, naming | [references/structure.md](references/structure.md) |
| Mocking, interfaces | [references/mocking.md](references/mocking.md) |

## Review Checklist

- [ ] Tests are table-driven with clear case names
- [ ] Subtests use t.Run for parallel execution
- [ ] Test names describe behavior, not implementation
- [ ] Errors include got/want with descriptive message
- [ ] Cleanup registered with t.Cleanup
- [ ] Parallel tests don't share mutable state
- [ ] Mocks use interfaces defined in test file
- [ ] Coverage includes edge cases and error paths

## Critical Patterns

### Table-Driven Tests

```go
// BAD - repetitive
func TestAdd(t *testing.T) {
    if Add(1, 2) != 3 {
        t.Error("wrong")
    }
    if Add(0, 0) != 0 {
        t.Error("wrong")
    }
}

// GOOD
func TestAdd(t *testing.T) {
    tests := []struct {
        name     string
        a, b     int
        want     int
    }{
        {"positive numbers", 1, 2, 3},
        {"zeros", 0, 0, 0},
        {"negative", -1, 1, 0},
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got := Add(tt.a, tt.b)
            if got != tt.want {
                t.Errorf("Add(%d, %d) = %d, want %d", tt.a, tt.b, got, tt.want)
            }
        })
    }
}
```

### Error Messages

```go
// BAD
if got != want {
    t.Error("wrong result")
}

// GOOD
if got != want {
    t.Errorf("GetUser(%d) = %v, want %v", id, got, want)
}

// For complex types
if diff := cmp.Diff(want, got); diff != "" {
    t.Errorf("GetUser() mismatch (-want +got):\n%s", diff)
}
```

### Parallel Tests

```go
func TestFoo(t *testing.T) {
    tests := []struct{...}

    for _, tt := range tests {
        tt := tt  // capture (not needed Go 1.22+)
        t.Run(tt.name, func(t *testing.T) {
            t.Parallel()
            // test code
        })
    }
}
```

### Cleanup

```go
// BAD - manual cleanup, skipped on failure
func TestWithTempFile(t *testing.T) {
    f, _ := os.CreateTemp("", "test")
    defer os.Remove(f.Name())  // skipped if test panics
}

// GOOD
func TestWithTempFile(t *testing.T) {
    f, _ := os.CreateTemp("", "test")
    t.Cleanup(func() {
        os.Remove(f.Name())
    })
}
```

## Anti-Patterns

### 1. Testing Internal Implementation

```go
// BAD - tests private state
func TestUser(t *testing.T) {
    u := NewUser("alice")
    if u.id != 1 {  // testing internal field
        t.Error("wrong id")
    }
}

// GOOD - tests behavior
func TestUser(t *testing.T) {
    u := NewUser("alice")
    if u.ID() != 1 {
        t.Error("wrong ID")
    }
}
```

### 2. Shared Mutable State

```go
// BAD - tests interfere with each other
var testDB = setupDB()

func TestA(t *testing.T) {
    t.Parallel()
    testDB.Insert(...)  // race!
}

// GOOD - isolated per test
func TestA(t *testing.T) {
    db := setupTestDB(t)
    t.Cleanup(func() { db.Close() })
    db.Insert(...)
}
```

### 3. Assertions Without Context

```go
// BAD
assert.Equal(t, want, got)  // "expected X got Y" - which test?

// GOOD
assert.Equal(t, want, got, "user name after update")
```

## When to Load References

- Reviewing test file structure → structure.md
- Reviewing mock implementations → mocking.md

## Review Questions

1. Are tests table-driven with named cases?
2. Do error messages include input, got, and want?
3. Are parallel tests isolated (no shared state)?
4. Is cleanup done via t.Cleanup?
5. Do tests verify behavior, not implementation?