Test Expert

This skill helps you generate robust, idiomatic test code and ensure it is placed in the correct project location following industry best practices.

Workflow

1. •Detect Environment: Identify language and framework (check pyproject.toml, package.json, go.mod).
2. •Determine Location:
   • •Python: tests/ (root level) or src/package/tests/.
   • •JS/TS: src/__tests__/ or *.test.ts collocated.
   • •Go: *_test.go collocated.
   • •Java/Kotlin: src/test/java/.
3. •Generate Test Case:
   • •AAA Pattern: Arrange, Act, Assert.
   • •Descriptive Names: test_should_return_404_when_user_not_found.
   • •Isolation: Mock external I/O (DB, Network).
4. •Verify: Run the tests to prove they fail (RED) and then pass (GREEN).

Deep Dive: Python (pytest)

Use pytest as the default unless strictly constrained to unittest.

1. The Basics

def test_add_should_sum_two_numbers():
    # Arrange
    a, b = 2, 3
    # Act
    result = add(a, b)
    # Assert
    assert result == 5  # No self.assertEqual needed

2. Parametrization (Don't write loops)

import pytest

@pytest.mark.parametrize("input_a, input_b, expected", [
    (1, 1, 2),
    (10, 20, 30),
    (-1, 1, 0),
])
def test_add_various_cases(input_a, input_b, expected):
    assert add(input_a, input_b) == expected

3. Fixtures (Setup/Teardown)

Use fixtures for reusable state. Scope them appropriately (function, module, session).

@pytest.fixture
def sample_user():
    return User(name="Alice", age=30)

def test_user_age(sample_user):
    assert sample_user.age == 30

4. Mocking (External Dependencies)

Use unittest.mock (standard lib) or pytest-mock fixture.

from unittest.mock import Mock

def test_api_call(mocker): # using pytest-mock
    # Arrange
    mock_api = mocker.patch("my_app.services.api_client.get")
    mock_api.return_value = {"status": "ok"}
    
    # Act
    result = fetch_data()
    
    # Assert
    assert result == "ok"
    mock_api.assert_called_once()

Deep Dive: TypeScript (Vitest/Jest)

1. Async & Spies

import { vi, describe, it, expect } from 'vitest';
import { UserService } from './user-service';

describe('UserService', () => {
  it('should save user', async () => {
    // Arrange
    const dbMock = { save: vi.fn().mockResolvedValue(true) };
    const service = new UserService(dbMock);
    
    // Act
    await service.register('alice');
    
    // Assert
    expect(dbMock.save).toHaveBeenCalledWith('alice');
  });
});

Quality Gate Checklist

• • Location: Is the file in the standard path?
• • Imports: Are source modules imported correctly?
• • Style: snake_case for Python tests, camelCase for JS/TS tests.
• • Behavior: Does it test behavior, not implementation details?