fastapi-dependency-injection

FastAPI's Depends pattern lets you define reusable dependencies as functions or classes, then inject them into endpoints by declaring them as parameters. Dependencies are executed once per request and their results are cached, reducing redundant database calls or authentication checks across multiple handlers.

Request-scoped dependencies execute and reset for each HTTP request, ideal for database sessions or user context. Application-scoped dependencies persist across requests, suited for connection pools or configuration. FastAPI's dependency system manages both via the Depends declaration and generator-backed cleanup.

Yes, sub-dependencies let you build chains where one dependency depends on another. FastAPI resolves the full chain automatically, allowing complex flows like authentication → user lookup → database session without manual nesting or redundant execution.

Use generator-backed dependencies with yield instead of return. Code after yield runs when the request ends, ensuring cleanup happens automatically. This pattern handles database disconnects, file closes, and external service teardown without try-finally boilerplate.

Dependency injection becomes valuable once you have multiple endpoints sharing logic like database access or authentication. For single-endpoint prototypes it's optional, but it scales modularity and testability as your API grows, making it a best practice even early.

Class-based dependencies use callable instances with __call__, offering statefulness and composability. Function-based dependencies are simpler for stateless logic. Both integrate identically with Depends; choose classes when you need initialization, validation, or method-based organization.