python-clean-architecture

Structure a FastAPI project using clean architecture by adopting a strict three-layer structure: Routers → Operations → Database. This canonical layout enforces separation of concerns and uses Protocol-based dependency injection to keep code scalable and testable.

Protocol-based dependency injection in Python uses structural subtyping to define interfaces without inheritance. Applying this in FastAPI decouples routers from concrete database operations, promoting testability and reducing coupling across architectural layers.

The best way to refactor existing Python code for testability is to apply clean architecture principles. Introduce Protocol-based dependency injection and migrate logic into a three-layer structure to reduce coupling, increase cohesion, and isolate business rules from infrastructure.

Yes, you can use Python design patterns like Protocol and Callable to decouple FastAPI endpoints. Defining use cases and repositories with Protocols enables data-first design and seamless dependency injection, keeping endpoint logic lightweight and isolated.

You should avoid a three-layer architecture for simple Python scripts or prototypes where strict separation of concerns introduces unnecessary boilerplate. Clean architecture patterns are designed for scalable, complex projects requiring long-term testability and maintainability.