12 - Testing and Code Quality

This track is about the discipline that lets you change code without fear. Everything you built in the earlier tracks made promises — this endpoint returns 201, this validator rejects bad input, this service raises a domain exception — and until those promises are pinned down by automated tests, the only thing verifying them is you, manually, once. This track turns those promises into executable checks and teaches you to write them at the right level: fast isolated unit tests at the base, real-database integration tests in the middle, a thin tip of end-to-end and contract tests, all driven by genuine TDD discipline, automated in CI, and backed by objective code-quality measurement.

How this track works

  • It assumes you've finished track 02 (API Layer and Request Handling) — you're comfortable with FastAPI, Pydantic, the handler → service → repository layering, dependency injection with Depends, and the OpenAPI spec FastAPI generates. This track tests exactly the kind of service you built there, and it leans on that layering constantly (a service that's cleanly separated from HTTP and the database is a service you can actually test).
  • Everything is built in Python with pytest as the runner, plus httpx/TestClient, testcontainers, pytest-mock, schemathesis, ruff, and mypy. You grow one testing-lab project across the modules and, in the capstone, bring a real earlier-track service to full pyramid coverage.
  • Every module builds on the ones before it — the test pyramid introduced in module 00 is the spine, and each later module fills in one layer or discipline (unit → TDD → integration → mocking → e2e/contract → quality → CI/strategy). Go in order; no forward references.
  • Each standard module has the same shape: why it matters, concepts, a command reference with real pytest/FastAPI code, progressive hands-on exercises (do them — including a "diagnose and fix" scenario each), an independent challenge with no code, common mistakes, and a checkpoint quiz. Two cumulative reviews (in modules 03 and 07) mix questions from everything so far, closed-book.
  • CI is covered deliberately briefly in module 07 — track 13 (DevOps for Backend Engineers) owns CI/CD, containers, and deployment in depth, and this track cross-references it rather than duplicating it.
  • The last module is an open-ended capstone with no solution given — it's the integration test for the whole track.

Modules

# Module What you'll be able to do Time
00 Testing fundamentals and the test pyramid Explain why we test, place tests on the unit/integration/e2e pyramid, tell the four test doubles apart, and run pytest 60-90 min
01 Unit testing in depth Use fixtures and parametrization, test pure vs. side-effecting code, and read coverage as a signal not a target 60-90 min
02 Test-driven development in practice Drive a feature with real red-green-refactor, triangulate, and judge when TDD helps vs. slows you down 60-90 min
03 Integration testing Test against a real test database, drive routes with TestClient/httpx, isolate test data, and use testcontainers 75-100 min
04 Mocking and dependency injection for testability Mock external services with FastAPI overrides and fakes, control time, and avoid over-mocking 60-90 min
05 E2E and contract testing Write critical-path e2e tests, verify the API against its OpenAPI spec with schemathesis, and judge when e2e is worth the cost 60-90 min
06 Code quality and static analysis Lint with ruff, type-check with mypy, measure complexity, wire pre-commit hooks, and reason about quality beyond style 60-90 min
07 Testing in CI and test strategy Run tests in CI, triage flaky tests, and steer a balanced pyramid as a codebase grows 60-90 min
08 Capstone project Bring an untested FastAPI service to full test-pyramid coverage with mocks, a real test DB, an e2e/contract test, and passing lint/type checks 4-6 hrs

Start here → 00-testing-fundamentals-and-the-test-pyramid/README.md

Back to the master index: ../README.md

Once you've completed this track, the next one is 13-devops-for-backend-engineers — it picks up exactly where module 07's CI section stops, taking the tests and quality gates you built here and connecting them to the full path to production: CI/CD pipelines, containers, and deployment strategies.