How to Implement Dependency Injection in FastAPI

FastAPI's dependency injection system uses the Depends() function to deliver reusable logic like authentication, database connections, and validation to path operations. Dependencies can be implemented as functions or classes and injected at three levels: individual endpoints, router groups, or globally across the application. Function dependencies work for most cases, while class dependencies suit stateful logic and object-oriented patterns. The system eliminates code repetition, enables modular architecture, and handles common tasks like authentication guards, logging, rate limiting, and configuration management. Dependencies can execute with or without returning values to path operations, making them flexible for both data retrieval and side-effects.