QA and Testing — Cheat Sheet¶

# tests/test_calculator.py

def test_add_two_numbers():
    # Arrange
    a, b = 2, 3
    # Act
    result = add(a, b)
    # Assert
    assert result == 5

def test_divide_by_zero_raises():
    with pytest.raises(ZeroDivisionError):
        divide(10, 0)

import pytest

@pytest.fixture
def db_connection():
    """Set up a test database connection."""
    conn = create_test_db()
    yield conn          # test runs here
    conn.close()        # teardown after test

@pytest.fixture(scope="module")
def expensive_resource():
    """Created once per module, shared across all tests in the module."""
    return load_large_dataset()

def test_query(db_connection):
    result = db_connection.execute("SELECT 1")
    assert result == 1

import pytest

@pytest.mark.parametrize("a,b,expected", [
    (2, 3, 5),
    (0, 0, 0),
    (-1, 1, 0),
    (100, -50, 50),
])
def test_add(a, b, expected):
    assert add(a, b) == expected

import pytest

@pytest.mark.slow
def test_large_dataset():
    ...

@pytest.mark.skip(reason="not implemented yet")
def test_future_feature():
    ...

@pytest.mark.xfail(reason="known bug #123")
def test_known_broken():
    ...

# tests/conftest.py — fixtures available to all tests in the directory

import pytest
from myapp import create_app, db

@pytest.fixture(scope="session")
def app():
    app = create_app(testing=True)
    yield app

@pytest.fixture
def client(app):
    return app.test_client()

@pytest.fixture(autouse=True)
def reset_db(app):
    """Automatically reset database before each test."""
    with app.app_context():
        db.create_all()
        yield
        db.drop_all()

from unittest.mock import MagicMock, patch, call

# 1. Create a mock object
mock_service = MagicMock()
mock_service.get_user.return_value = {"id": 1, "name": "Alice"}

result = process_user(1, service=mock_service)

# Verify calls
mock_service.get_user.assert_called_once_with(1)
mock_service.get_user.assert_called_with(1)  # most recent call
mock_service.get_user.assert_any_call(1)      # any call with these args

# 2. Patch a module-level name
with patch('mymodule.send_email') as mock_email:
    register_user("alice@example.com")
    mock_email.assert_called_once()

# 3. Patch as a decorator
@patch('mymodule.requests.get')
def test_fetch_user(mock_get):
    mock_get.return_value.json.return_value = {"id": 1}
    result = fetch_user(1)
    assert result["id"] == 1

# 4. Mock raising an exception
mock_db = MagicMock()
mock_db.query.side_effect = ConnectionError("DB down")

# 5. Spy on a real object (wrap it)
from unittest.mock import wraps
real_calculator = Calculator()
spy = MagicMock(wraps=real_calculator)
spy.add(2, 3)  # calls the real method
spy.add.assert_called_once_with(2, 3)