Test Expert

        /\
       /  \  E2E Tests (Few)
      /____\
     /      \  Integration Tests (Some)
    /________\
   /          \  Unit Tests (Many)
  /__________\

def test_add_numbers():
    assert add(2, 3) == 5  # Function doesn't exist yet

def add(a, b):
    return a + b

def add(a: int, b: int) -> int:
    """Add two numbers and return the result."""
    return a + b

# Good: Clear, focused, independent
def test_user_can_be_created_with_email():
    # Arrange
    email = "user@example.com"

    # Act
    user = User(email=email)

    # Assert
    assert user.email == email
    assert user.is_active == True

# Good names describe what's being tested
def test_user_creation_with_valid_email_succeeds():
    pass

def test_user_creation_with_invalid_email_raises_error():
    pass

def test_empty_cart_has_zero_total():
    pass

import pytest
from myapp import Calculator

class TestCalculator:
    @pytest.fixture
    def calc(self):
        return Calculator()

    def test_add(self, calc):
        assert calc.add(2, 3) == 5

    def test_divide_by_zero_raises_error(self, calc):
        with pytest.raises(ZeroDivisionError):
            calc.divide(10, 0)

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

describe('Calculator', () => {
  let calc;

  beforeEach(() => {
    calc = new Calculator();
  });

  test('adds two numbers', () => {
    expect(calc.add(2, 3)).toBe(5);
  });

  test('throws error on division by zero', () => {
    expect(() => calc.divide(10, 0)).toThrow();
  });

  test.each([
    [2, 3, 5],
    [0, 0, 0],
    [-1, 1, 0],
  ])('add(%i, %i) returns %i', (a, b, expected) => {
    expect(calc.add(a, b)).toBe(expected);
  });
});

#!/usr/bin/env bats

@test "script exits with status 0 on success" {
  run ./myscript.sh input.txt
  [ "$status" -eq 0 ]
}

@test "script produces expected output" {
  run ./myscript.sh input.txt
  [ "${lines[0]}" = "Expected output" ]
}

@test "script fails with invalid input" {
  run ./myscript.sh nonexistent.txt
  [ "$status" -ne 0 ]
  [[ "$output" =~ "Error" ]]
}

from unittest.mock import Mock, patch, MagicMock

# Mock an object
mock_db = Mock()
mock_db.get_user.return_value = {"id": 1, "name": "Test"}

# Patch a function
@patch('myapp.external_api_call')
def test_function(mock_api):
    mock_api.return_value = {"status": "success"}
    result = my_function()
    assert result == expected
    mock_api.assert_called_once_with(expected_arg)

// Jest mocking
jest.mock('./api');
import { fetchUser } from './api';

test('loads user data', async () => {
  fetchUser.mockResolvedValue({ id: 1, name: 'Test' });

  const user = await loadUser(1);

  expect(user.name).toBe('Test');
  expect(fetchUser).toHaveBeenCalledWith(1);
});

import pytest
from myapp import create_app, db

@pytest.fixture
def app():
    app = create_app('testing')
    with app.app_context():
        db.create_all()
        yield app
        db.session.remove()
        db.drop_all()

def test_user_can_be_saved_to_database(app):
    user = User(email='test@example.com')
    db.session.add(user)
    db.session.commit()

    retrieved = User.query.filter_by(email='test@example.com').first()
    assert retrieved is not None
    assert retrieved.email == 'test@example.com'

def test_api_returns_user_list(client):
    response = client.get('/api/users')

    assert response.status_code == 200
    assert len(response.json) > 0
    assert 'email' in response.json[0]