Perfect — let’s elevate the testing quality.

Below is a clean, idiomatic, production-style unit test suite for Week 3 concepts. This reflects:

• Table-driven tests
• Proper error checking (errors.Is, errors.As)
• No string comparison for errors
• Clean test naming
• No unnecessary logging
• Focused assertions

---

🧪 service.go (Reference Implementation Under Test)

package service

import (
	"errors"
	"fmt"
)

var ErrStorageFailure = errors.New("storage failure")

type Storage interface {
	Save(data string) error
}

type ValidationError struct {
	Field   string
	Message string
}

func (v ValidationError) Error() string {
	return fmt.Sprintf("validation failed on %s: %s", v.Field, v.Message)
}

type UserService struct {
	storage Storage
}

func NewUserService(s Storage) *UserService {
	return &UserService{storage: s}
}

func (u *UserService) CreateUser(name string) error {
	if name == "" {
		return ValidationError{
			Field:   "name",
			Message: "cannot be empty",
		}
	}

	if err := u.storage.Save(name); err != nil {
		return fmt.Errorf("failed to create user: %w", err)
	}

	return nil
}

---

🧪 service_test.go (Clean, Idiomatic Tests)

package service

import (
	"errors"
	"testing"
)

---

🔹 Mock Storage (Clean & Minimal)

type mockStorage struct {
	shouldFail bool
}

func (m *mockStorage) Save(data string) error {
	if m.shouldFail {
		return ErrStorageFailure
	}
	return nil
}

---

1️⃣ Test Success Path

func TestCreateUser_Success(t *testing.T) {
	storage := &mockStorage{shouldFail: false}
	service := NewUserService(storage)

	err := service.CreateUser("Aditya")
	if err != nil {
		t.Fatalf("expected no error, got %v", err)
	}
}

Why this is clean:

• No unnecessary checks
• Fails fast
• Clear intent

---

2️⃣ Test Validation Error (Using errors.As)

func TestCreateUser_ValidationError(t *testing.T) {
	storage := &mockStorage{}
	service := NewUserService(storage)

	err := service.CreateUser("")
	if err == nil {
		t.Fatal("expected validation error, got nil")
	}

	var ve ValidationError
	if !errors.As(err, &ve) {
		t.Fatalf("expected ValidationError, got %T", err)
	}

	if ve.Field != "name" {
		t.Fatalf("expected field 'name', got %s", ve.Field)
	}
}

Why this is idiomatic:

• Uses errors.As
• Does not compare strings
• Verifies structured data

---

3️⃣ Test Wrapped Storage Error (Using errors.Is)

func TestCreateUser_StorageFailure(t *testing.T) {
	storage := &mockStorage{shouldFail: true}
	service := NewUserService(storage)

	err := service.CreateUser("Aditya")
	if err == nil {
		t.Fatal("expected error, got nil")
	}

	if !errors.Is(err, ErrStorageFailure) {
		t.Fatalf("expected wrapped ErrStorageFailure, got %v", err)
	}
}

Why this is correct:

• Uses errors.Is
• Does not rely on error string
• Validates wrapping behavior

---

4️⃣ Table-Driven Test (Professional Pattern)

func TestCreateUser_TableDriven(t *testing.T) {
	tests := []struct {
		name        string
		input       string
		shouldFail  bool
		expectError bool
	}{
		{
			name:        "valid user",
			input:       "Aditya",
			shouldFail:  false,
			expectError: false,
		},
		{
			name:        "empty name",
			input:       "",
			shouldFail:  false,
			expectError: true,
		},
		{
			name:        "storage failure",
			input:       "Aditya",
			shouldFail:  true,
			expectError: true,
		},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			storage := &mockStorage{shouldFail: tt.shouldFail}
			service := NewUserService(storage)

			err := service.CreateUser(tt.input)

			if tt.expectError && err == nil {
				t.Fatal("expected error, got nil")
			}

			if !tt.expectError && err != nil {
				t.Fatalf("unexpected error: %v", err)
			}
		})
	}
}

Why this is senior-level:

• Scalable
• Clean test names
• No duplicated logic
• Clear scenario separation

---

🧠 What Makes These Tests “Clean”

✔ Small, focused

Each test verifies one behavior.

✔ No string comparison of errors

We use:

• errors.Is
• errors.As

✔ No unnecessary logging

Only fail when necessary.

✔ No shared mutable global state

✔ Proper naming convention

TestFunction_Scenario

---

🎯 What Student Learns From This

• Interface-based mocking
• Structured error testing
• Proper use of errors.Is and errors.As
• Table-driven test design
• Writing maintainable tests

This is exactly how Go is tested in real-world backend systems.

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