Fastapi
211 bites tagged Fastapi: interview questions with model answers, and 60-second explainers.
Test yourself: Top 30 Fastapi interview questions →Multiple choice, with the correct answer and why it is correct on every question. Free, no sign-in.Choosing Uvicorn worker count in production
Workers exist to use multiple CPU cores past the GIL; a common starting point ties count to cores, then you tune by load testing, balancing CPU and memory (each worker is a full copy) against connection-pool…
Reading the full response body in middleware
Responses stream as multiple body messages and headers go first, so you cannot add a header after seeing the body; you must buffer all chunks, compute the hash, set the header, then resend.
OAuth2 social login with your own JWT
A login endpoint redirects to the provider with a state param, a callback exchanges the code for the provider token, you fetch the user profile, upsert the local user, then mint your own JWT.
selectinload vs joinedload for eager loading
Use eager loading options to avoid lazy N+1; joinedload uses a single JOIN (good for many-to-one) but can fan out rows on collections; selectinload issues a second IN query (better for one-to-many).
Feature-based vs layer-based project structure
Layer-based groups by technical role and is simple early but scatters a feature across folders; feature-based groups by domain, improving cohesion and ownership at the cost of some duplication and…
Streaming large file downloads efficiently
Use StreamingResponse with a generator that yields chunks (or FileResponse for an on-disk file), set media_type and a Content-Disposition header, so memory stays flat regardless of file size.
Pydantic computed fields in response models
A @computed_field decorated property is excluded from input and validation but included in serialization and the OpenAPI schema, ideal for values like full_name derived from other fields.
Mapping camelCase JSON to snake_case Pydantic fields
Set an alias_generator (to_camel) plus populate_by_name in model config, accept aliases on input, and serialize with by_alias=True so responses come out camelCase.
Pydantic BaseModel vs dataclasses in FastAPI
BaseModel validates and coerces data at runtime, parses and serializes JSON, integrates with OpenAPI schema generation, and supports rich validators; dataclasses only store data with no validation.
Testing a DB endpoint via dependency override
Use app.dependency_overrides to swap the real get_db for one yielding a test database session, run against a disposable SQLite or test Postgres, and assert through TestClient.
Secure refresh token flow for access renewal
Short-lived access token, longer-lived refresh token stored server-side, a refresh endpoint that validates and rotates the refresh token issuing a new pair.
Zero-downtime blue-green deploys on Kubernetes
Run blue and green deployments, switch a Service or ingress selector to the new color after readiness probes pass, drain old pods gracefully, and handle backward-compatible DB migrations.
Limits of BackgroundTasks vs Celery or ARQ
BackgroundTasks run in the same worker with no persistence, retries, or visibility, and die with the process; Celery or ARQ add durability, retries, scheduling, and separate workers.
Diagnosing a slow FastAPI endpoint under load
Add timing and tracing to isolate the slow span, watch CPU vs wait time and event-loop lag, then use profilers like py-spy or cProfile and DB EXPLAIN.
WebSocket connection manager and broadcast
A manager class holding a list of active connections, connect accepts and appends, disconnect removes, broadcast iterates sending to each, all wrapped in try/finally to handle disconnects.
BackgroundTasks dependency vs response.background
The injected BackgroundTasks parameter lets endpoints and dependencies stack tasks and is the common path; response.background attaches a single Starlette task when you return a Response object…
Propagating a correlation ID without parameter passing
Middleware reads or generates the header, stores it in a contextvars.ContextVar, service code reads it anywhere, and logging filters inject it.
Revoking stateless JWTs on logout
A server-side denylist of revoked token IDs checked per request, or short-lived access tokens paired with revocable refresh tokens.
SQLAlchemy connection pooling across Uvicorn workers
Each worker has its own pool; total DB connections equal workers times (pool_size plus max_overflow); overflow connections are temporary; misconfiguration exhausts DB…
MongoDB async ODM vs SQLAlchemy sessions
Motor client as a connection pool with no SQLAlchemy-style session or transaction; Beanie document models over Motor; initialize once at startup and await find queries.
Explain multi-stage Docker builds for Python and builder vs runtime
Tests separation of build-time and runtime concerns. A strong answer contrasts the builder stage (gcc, headers, wheels) with the runtime stage (slim base, copied artifacts, no compiler). Red flag: citing size alone while ignoring security and caching.
How do you manage configuration and secrets for a containerized FastAPI app?
Tests 12-factor config separation and Docker secret hygiene. A strong answer uses pydantic-settings with runtime env vars, lru_cache, and keeps .env out of the image. Red flag: baking credentials into Dockerfile layers or committing .env files.
What are the key responsibilities for Nginx versus Uvicorn?
This tests the reverse-proxy versus ASGI-server boundary. A strong answer gives Nginx TLS termination, static files, buffering, and load balancing, while Uvicorn runs the Python app and async workers.
Why use an ASGI server like Uvicorn instead of the dev server?
Tests: dev vs prod environment distinction. Answer: Uvicorn is a prod server program; contrast dev server's constant restart/break/fix cycle with prod needs for performance, stability, and uninterrupted access.
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