congiuluc/testing-backend-python

Backend Testing (Python)

When to Use

• Creating unit tests for services, validators, or domain logic
• Writing integration tests for FastAPI endpoints
• Mocking repository or external service dependencies
• Testing middleware or dependency overrides

Official Documentation

• pytest
• pytest-asyncio
• pytest-mock
• httpx AsyncClient
• FastAPI Testing
• factory_boy

Core Rule

Every feature must have corresponding tests before it is considered complete.

Procedure

1. Identify test type: unit test (service logic) or integration test (API endpoint)
2. Follow naming conventions in Python testing reference
3. Review service test sample
4. Use Arrange-Act-Assert pattern for all tests
5. Mock external dependencies at boundaries with pytest-mock or unittest.mock
6. Use plain assert with descriptive messages
7. Cover: happy path, edge cases, error scenarios, validation
8. Verify tests are deterministic — no external service or time dependencies
9. Run pytest locally before committing

Test Structure

tests/
  unit/
    domain/                # Domain model unit tests
    application/           # Service unit tests
    infrastructure/        # Repository unit tests
  integration/             # API endpoint integration tests
  conftest.py              # Shared fixtures

Naming Convention

• Test file: test_{module_name}.py (e.g., test_product_service.py)
• Test class: Test{ClassUnderTest} (e.g., TestProductService)
• Test function: test_should_do_x_when_condition (e.g., test_should_return_product_when_id_exists)