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Module 08 / Project 05 — Integration Testing

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Focus

  • Testing a FastAPI application with TestClient
  • Full request/response cycle testing (create, read, update, delete)
  • Testing error cases: 404, invalid input
  • conftest.py fixtures for fresh app instances

Why this project exists

Unit tests check individual functions in isolation. Integration tests check that the whole system works together — that HTTP requests reach the right endpoint, data is stored and retrieved correctly, and error responses have the right status codes. This project gives you a simple FastAPI todo API and teaches you to test it from the outside, the same way a real client would use it. This is the closest thing to "clicking through the app" but automated and repeatable.

Run

cd projects/modules/08-testing-advanced/05-integration-testing
pytest tests/ -v

Expected output

tests/test_integration.py::test_list_todos_empty PASSED
tests/test_integration.py::test_create_todo PASSED
tests/test_integration.py::test_create_todo_returns_id PASSED
tests/test_integration.py::test_get_todo_not_found PASSED
tests/test_integration.py::test_create_and_retrieve_todo PASSED
tests/test_integration.py::test_delete_todo PASSED
tests/test_integration.py::test_delete_nonexistent_todo PASSED
tests/test_integration.py::test_create_todo_invalid_input PASSED
tests/test_integration.py::test_full_crud_flow PASSED

All tests should pass. Each test gets a fresh app instance so they do not interfere with each other.

Alter it

  1. Add a PUT /todos/{todo_id} endpoint that updates a todo's title and done status. Write integration tests for it.
  2. Add a test that creates five todos and verifies that GET /todos returns all five.
  3. Add a GET /todos?done=true query parameter that filters completed todos. Write tests for the filter.

Break it

  1. Remove the conftest.py file and import TestClient directly in each test without the fixture. Create a todo in one test and try to read it in another. What happens?
  2. Change the POST /todos endpoint to not return the created todo. What tests break?
  3. Change the todo ID generation to always return 1. What tests break?

Fix it

  1. Put conftest.py back and verify each test gets an isolated app instance.
  2. Restore the POST response to include the full todo object.
  3. Fix the ID generation so each todo gets a unique ID.

Explain it

  1. What is the difference between a unit test and an integration test?
  2. Why does each test need a fresh app instance (a fresh client fixture)?
  3. What does TestClient do under the hood — does it start a real server?
  4. Why do we test error cases (404, invalid input) and not just the happy path?

Mastery check

You can move on when you can:

  • Write a TestClient-based integration test from memory.
  • Test the full CRUD cycle (create, read, update, delete) for any resource.
  • Test error responses and verify status codes.
  • Explain why test isolation matters for integration tests.

Next

Congratulations — you have completed Module 08. You now have a professional-grade testing toolkit: parametrize for coverage, mocking for isolation, fixtures for setup, Hypothesis for edge cases, and integration tests for confidence. Go back to Module Index and pick your next module.