How do you declare a function as a dependency, and why?

Tests FastAPI dependency injection mechanics and the exact role of the Depends callable. Strong answer: write a dependency function, import Depends from fastapi, and add it to path operation parameters so FastAPI resolves it automatically. The purpose is inversion of control: routes declare what they need and the framework injects it, keeping endpoints thin and shared logic reusable across the app. Red flag: calling the dependency manually inside the route or confusing it with middleware.