Top 10 FastAPI interview questions for senior roles

Why it exists

FastAPI became a default choice for building Python APIs, so interview panels needed a way to separate candidates who can copy a tutorial from candidates who understand what the framework is actually doing underneath. A generic Python or REST question does not surface that gap. A pointed FastAPI question does, because answering it well requires understanding async concurrency, validation, and dependency wiring, not just decorator syntax.

The mental model

Treat FastAPI as a thin, typed layer sitting on two pillars: Starlette underneath for ASGI concurrency, and Pydantic underneath for data validation, with a dependency injection system as the glue that wires request data, services, and database sessions together. Almost every hard interview question is really a question about one of these three pillars wearing a different costume.

How it works

Route handlers declared with async def run directly on the single event loop, so any blocking call inside one stalls every other request sharing that worker. Route handlers declared with plain def are automatically offloaded to a worker thread pool, which is why FastAPI can safely mix in old style blocking libraries as long as they stay behind def. Pydantic models attached to request and response types do double duty: they validate and coerce incoming data, serialize outgoing data, and generate the OpenAPI schema from the same definitions. Depends builds a resolvable graph of dependencies per request, dependencies written with yield run their teardown code after the response is sent, and overriding a dependency in tests swaps a real database session for a fake one without touching route code.

When it matters

Senior level questions target production failure modes: what happens under fifty concurrent requests if a blocking database driver is called from an async def route, why background tasks are not a substitute for a real queue when a job needs retries or must survive a process restart, and how many workers a deployment actually needs given FastAPI's single threaded event loop per process. The footgun is a candidate who can build a working CRUD app but has never reasoned about what blocks the event loop.

A concrete example

An interviewer asks what happens when an async def endpoint calls a synchronous, blocking SOAP client under fifty concurrent requests. A strong answer explains that the blocking call runs on the event loop itself, freezing every other request on that worker for its duration, then names two fixes: switch the route to plain def so FastAPI offloads it to a thread pool, or replace the SOAP client with an async one so the wait releases the loop.