"What's happening here: Type-safe - Your editor knows repo is a SnippetRepository. Full autocomplete, type checking works. Automatic cleanup - The context manager ensures the database session closes, even if an exception occurs. No global state - Every request gets its own session. No risk of one request interfering with another. Testable - Here's the magic: You override the dependency with an in-memory implementation. Your test doesn't hit the database."
"This is how professional FastAPI applications are structured: → Dependencies are functions that provide resources → FastAPI calls them automatically for each request → Tests override dependencies with test doubles → Your endpoint code stays clean - it just declares what it needs I see too many FastAPI tutorials that skip dependency injection or treat it as "advanced." However, this is foundational knowledge. If you're not leveraging Depends(), you're making testing harder than it needs to be."
FastAPI's Depends provides type-safe dependency injection so editors recognize concrete types, enabling autocomplete and static type checking. Dependencies can manage resources via context managers to ensure automatic cleanup of database sessions even on exceptions. Each request receives its own session, avoiding global state and cross-request interference. Tests replace dependencies with in-memory implementations, avoiding real databases, Docker, or fixture-based cleanup and producing fast tests. Professional FastAPI structure treats dependencies as functions that supply resources and lets the framework inject them per-request while tests supply test doubles. Dependency injection aligns with clean architecture and simplifies endpoint code.
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