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Implement a custom exception handler to catch ItemNotFoundError and return 404
Tests FastAPI exception handler registration beyond HTTPException. A strong answer covers creating a custom exception, using app.exception_handler, and returning a JSONResponse with status 404 and a structured body. Red flag: per-route try/except, plain dict.
Pydantic: Configuring Models with `model_config`
Think of model_config as the settings panel for your Pydantic models, letting you change validation rules like string length or immutability. Use it to enforce global constraints or make models immutable. The footgun is using the old class Config: from V1.
How would you use BackgroundTasks to run work after returning a 201?
What it tests: FastAPI deferred execution and failure modes. A strong answer injects BackgroundTasks, adds the task, returns 201, and notes same-process post-response execution with no persistence. Red flag: Treating it as a distributed queue like Celery.
Access raw request bytes in FastAPI for webhook verification
Tests FastAPI's Starlette integration and stream semantics. Outline: inject Request and await request.body, but the stream is single-use so JSON parsing later fails and docs are lost. Red flag: suggesting a Pydantic model still works after consuming the body.
Pydantic Computed Fields: Serialize Derived Values
A Pydantic computed field makes a derived value, like an area from width and length, part of your model's serialized output. Use it to include calculated attributes when calling .model_dump().
How do you declare a function as a dependency, and why?
Tests FastAPI dependency injection basics. Answer: create a function, import Depends, and add it to path operation parameters so FastAPI injects it. Purpose: routes declare what they need instead of hard-coding shared logic.
What is the purpose of yield in a dependency function?
Tests teardown logic in FastAPI dependencies. Yield splits setup from cleanup: code before yield runs pre-request, after yield runs post-response to close resources like DB sessions. Red flag: confusing it with return or thinking yield is only for generators.
How do you apply a dependency to an APIRouter without per-endpoint signatures?
Pass Depends() to APIRouter dependencies parameter; runs before every route in that router and shows in docs.
FastAPI: Handling Form Data, Not Just JSON
FastAPI can handle classic HTML form data, not just JSON. Use Form to define expected fields in your endpoint, just like query parameters. It's ideal for login pages. The footgun: forgetting to pip install python-multipart will break form parsing.
How would you override a FastAPI dependency during testing?
Tests your grasp of FastAPI's dependency override mechanism. A strong answer mentions app.dependency_overrides, notes that sub-dependencies are bypassed, and stresses clearing overrides after each test.
Declaring Request Headers in FastAPI
Treat request headers like any other parameter in FastAPI. Declare them in your function signature to access values like User-Agent or X-Token. FastAPI automatically converts hyphens to underscores, so User-Agent is accessed via the user_agent…
How do you use a Python class as a FastAPI dependency?
It tests whether you understand FastAPI DI beyond functions and when stateful encapsulation wins. Explain that Depends takes callable classes, centralizing setup and shared state in __init__. Red flag: claiming classes are pure syntactic sugar.
FastAPI: Reading Request Cookies
FastAPI treats request cookies like any other parameter. Declare them directly in your endpoint's function signature using Cookie(), and the framework will extract the value for you. Use this for reading session IDs or user preferences.
How does FastAPI execute setup and teardown in nested yield dependencies?
It tests FastAPI's dependency injection lifecycle and stack-like teardown for nested yield dependencies. Setup runs top-down; teardown runs bottom-up after the response. A red flag is claiming teardown order is arbitrary or follows garbage collection.
FastAPI: Returning HTML with HTMLResponse
Override FastAPI's default JSON output by using HTMLResponse to return a raw HTML string directly from an endpoint. It's for simple status pages or server-side rendered components.
How does FastAPI cache dependencies within a single request?
Tests if you know FastAPI caches a dependency after the first call in a request and reuses it across the tree. A strong answer covers default use_cache=True, request-scoped lifetime, and disabling it.
How would you implement a dependency requiring multi-source parameters?
Tests if you know FastAPI resolves dependency params like endpoint params. Great answers annotate each parameter with its source inside the dependency so FastAPI injects them independently. Red flag: manually parsing Request or merging values in the endpoint.
FastAPI: Use UploadFile for Efficient File Uploads
FastAPI handles file uploads as 'form data', giving you a streamable UploadFile object instead of a raw byte blob. Use this for endpoints like image or document submissions. The footgun is reading large files into memory instead of streaming them.
How does lifecycle differ for global vs path operation dependencies?
Global deps run on every request to any route; path-local deps run only for that route; expensive setup belongs in a lifespan event or cached singleton, not a dependency.
Explain the internal role of the Depends class
A strong answer notes it marks parameters for solver, enables recursive sub-dependencies and Annotated sharing, and feeds OpenAPI.