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Module · 5 lessons

Structure, Dependencies, and Middleware

Organize a growing API: share logic with dependency injection, split routes across files with routers, and run cross-cutting code with middleware and CORS.

Start module Back to FastAPI: Build Production APIs in Python

At a glance

Level
Intermediate
Lessons
5 lessons
Time to complete
1 week
Cost
Free forever · no sign-up

Welcome to Structure, Dependencies, and Middleware, the fourth module of the course. Your Task Manager works, but so far it lives in one file with logic copy-pasted across endpoints. Real applications grow — more routes, shared rules, repeated setup — and without structure they become tangled fast. This module teaches the three tools FastAPI gives you to stay organized at any size.

You’ll learn dependency injection with Depends — the elegant way to share logic (pagination, authentication, database sessions) across endpoints without repeating yourself — including class dependencies, sub-dependencies, and yield for setup-and-teardown. You’ll split a sprawling app into clean files with APIRouter, and run code on every request with middleware (timing, logging, custom headers) and enable browser clients with CORS. The module ends with a guided project: reorganizing the Task Manager into a modular, multi-router app with shared dependencies.

Every example is real, runnable code verified end to end. Start with Lesson 1, where you’ll meet dependency injection.

Lessons in this module

1 Dependency Injection Share logic across endpoints without repeating yourself: write a dependency function once and inject it anywhere with Depends. 2 Reusable and Sub-Dependencies Level up dependencies: use classes for typed dependencies, build dependencies on top of other dependencies, and manage setup and teardown with yield. 3 Bigger Applications with Routers Split a growing API across files with APIRouter: group related routes, give them a shared prefix and tags, and wire them into the app with include_router. 4 Middleware and CORS Run code on every request with middleware — timing, logging, headers — and let browser apps call your API by enabling CORS for the origins you trust. 5 Guided Project: Modular App Reorganize the Task Manager into a clean, modular app: routers in their own files, a shared dependency, and middleware plus CORS — verified end to end.
Achievement

Complete all 5 lessons to finish the Structure, Dependencies, and Middleware module.

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