FastAPI Route Builder

FastAPI is a modern Python web framework for building APIs, built on Starlette and Pydantic. It's one of the fastest Python frameworks available (on par with NodeJS/Go for async workloads). Key advantages: automatic OpenAPI/Swagger docs generation, type-safe request/response validation via Pydantic, native async support, and Python type hints for editor autocomplete.

These are HTTP methods that map to CRUD operations. GET retrieves data (read-only, no body). POST creates new resources (body with data). PUT replaces an entire resource. PATCH partially updates a resource. DELETE removes a resource. FastAPI decorators (@app.get, @app.post, etc.) bind a function to a specific method + path combination.

A Pydantic model is a Python class that inherits from BaseModel and defines the shape of request/response data with type annotations. FastAPI uses these for automatic validation and serialization. If a request body doesn't match the model, FastAPI returns a 422 error with details about what's wrong — without any extra validation code.

Dependency injection (DI) in FastAPI uses the Depends() function to declare shared logic — database sessions, authentication, query parameter parsing — that gets injected into route functions automatically. It avoids code duplication and makes routes testable. Example: def get_db() returns a DB session; routes declare db: Session = Depends(get_db).

Path parameters are part of the URL path: /users/{user_id} — FastAPI extracts them from the URL. Query parameters come after '?': /items?skip=0&limit=10 — FastAPI reads them from the query string. Both are type-validated. Path params are required; query params can be optional with default values.

The most common patterns: OAuth2 with password flow (built-in FastAPI OAuth2PasswordBearer), API keys in headers (x-api-key), or JWT tokens. Use Depends() to inject an auth check into any route. FastAPI includes HTTPBearer and APIKeyHeader security schemes that integrate with the auto-generated Swagger UI.