Unit Testing Best Practices Thực Sự Quan Trọng

# Arrange - Setup objects, data, và precondition
def test_apply_discount_reduces_price():
    original_price = 100.0
    discount_percent = 20
 
    # Act - Chạy function cần test
    result = apply_discount(original_price, discount_percent)
 
    # Assert - Verify kết quả
    assert result == 80.0

# ❌ Tệ — test quá nhiều thứ cùng lúc
def test_user_registration():
    user = register_user("alice", "[email protected]", "password123")
    assert user.username == "alice"
    assert user.email == "[email protected]"
    assert user.is_active is True
    assert user.role == "member"
    assert len(user.permissions) == 3
    assert user.created_at is not None

# ✅ Tốt — tách test cho từng concern
def test_register_user_sets_username():
    user = register_user("alice", "[email protected]", "password123")
    assert user.username == "alice"
 
def test_register_user_is_active_by_default():
    user = register_user("alice", "[email protected]", "password123")
    assert user.is_active is True
 
def test_register_user_has_member_role():
    user = register_user("alice", "[email protected]", "password123")
    assert user.role == "member"

# ❌ Tệ — các test share state qua global list
cart_items = []
 
def test_add_item():
    cart_items.append("laptop")
    assert len(cart_items) == 1
 
def test_add_another_item():
    cart_items.append("mouse")
    assert len(cart_items) == 2  # Phụ thuộc vào test_add_item chạy trước!

# ✅ Tốt — mỗi test tạo state riêng
def test_add_item():
    cart = ShoppingCart()
    cart.add("laptop")
    assert cart.count() == 1
 
def test_add_another_item():
    cart = ShoppingCart()
    cart.add("mouse")
    assert cart.count() == 1  # Independent, deterministic

from unittest.mock import Mock
 
def test_get_user_profile_returns_formatted_name():
    # Arrange — mock database
    mock_db = Mock()
    mock_db.find_user.return_value = {"first_name": "Alice", "last_name": "Smith"}
 
    service = UserService(db=mock_db)
 
    # Act
    profile = service.get_profile(user_id=42)
 
    # Assert
    assert profile.display_name == "Alice Smith"
    mock_db.find_user.assert_called_once_with(42)

# ❌ Tệ — không nói lên gì
def test1():
    ...
 
def test_login():
    ...
 
# ✅ Tốt — đọc như specification
def test_login_with_empty_password_returns_error():
    ...
 
def test_login_with_expired_token_redirects_to_signin():
    ...
 
def test_calculate_tax_with_zero_amount_returns_zero():
    ...

✅ test_login_with_valid_credentials_returns_token
✅ test_login_with_wrong_password_returns_401
❌ test_login_with_expired_token_redirects_to_signin
✅ test_login_with_locked_account_returns_403

# ❌ Tệ — test implementation detail nội bộ
def test_user_service_internal_hash_password():
    service = UserService()
    hashed = service._hash_password("secret")  # Truy cập private method
    assert hashed.startswith("$2b$")
 
# ✅ Tốt — test behavior qua public API
def test_user_service_create_user_stores_hashed_password():
    service = UserService(db=mock_db)
    service.create_user("alice", "secret")
 
    stored_password = mock_db.save_user.call_args[0][1]
    assert stored_password != "secret"  # Đã hash, không phải plaintext

# ❌ Integration test giả vờ là unit test
def test_get_weather():
    result = weather_service.get_current("Ho Chi Minh City")  # Gọi API thật
    assert result["temperature"] > 0
 
# ✅ Unit test thật — mock HTTP call
def test_get_weather_parses_response():
    mock_client = Mock()
    mock_client.get.return_value = {"temp": 32, "unit": "celsius"}
 
    service = WeatherService(client=mock_client)
    result = service.get_current("Ho Chi Minh City")
 
    assert result["temperature"] == 32

# ❌ Cần FastAPI chạy để test business logic
from fastapi.testclient import TestClient
from main import app
 
def test_discount_calculation():
    client = TestClient(app)
    response = client.post("/api/discount", json={"price": 100, "percent": 20})
    assert response.json()["final_price"] == 80.0
 
# ✅ Test logic trực tiếp — không cần framework
def test_discount_calculation():
    result = calculate_discount(price=100, percent=20)
    assert result == 80.0

def test_transfer_money_succeeds():
    sender = Account(balance=1000)
    receiver = Account(balance=500)
 
    transfer(sender, receiver, amount=200)
 
    assert sender.balance == 800
    assert receiver.balance == 700

def test_transfer_exact_balance_succeeds():
    sender = Account(balance=100)
    receiver = Account(balance=0)
 
    transfer(sender, receiver, amount=100)
 
    assert sender.balance == 0
    assert receiver.balance == 100
 
def test_transfer_zero_amount_does_nothing():
    sender = Account(balance=100)
    receiver = Account(balance=50)
 
    transfer(sender, receiver, amount=0)
 
    assert sender.balance == 100
    assert receiver.balance == 50

import pytest
 
def test_transfer_insufficient_funds_raises_error():
    sender = Account(balance=50)
    receiver = Account(balance=100)
 
    with pytest.raises(InsufficientFundsError):
        transfer(sender, receiver, amount=200)
 
def test_transfer_negative_amount_raises_error():
    sender = Account(balance=100)
    receiver = Account(balance=50)
 
    with pytest.raises(ValueError):
        transfer(sender, receiver, amount=-50)