Easy everything in Backend Dev, page 5
Frontend on localhost:3000 gets errors calling FastAPI on localhost:8000. Name and fix?
This tests whether different ports mean different origins, causing CORS errors. A strong answer names CORS, notes ports are distinct origins, and outlines using CORSMiddleware with allow_origins.
How should you store user passwords in a database?
Tests knowledge of slow salted hashing versus encryption. Strong answers pick Argon2id or bcrypt, require unique per-user salts, describe verification via re-hashing with constant-time comparison, and cite bcrypt or argon2-cffi.
What are the three components of a JWT?
Tests if you know JWT structure beyond library usage. A strong answer lists header, payload, and signature; notes Base64Url encoding; and gives a registered claim like exp. A red flag is confusing signing with encryption.
How do you protect a FastAPI endpoint using Depends and OAuth2PasswordBearer?
OAuth2PasswordBearer sets the token URL, Depends injects it into the endpoint, and FastAPI validates the Bearer header.
Explain SQLAlchemy ORM vs Pydantic models in FastAPI
This tests separation of database schema from API contracts. A strong answer distinguishes SQLAlchemy table rows from Pydantic validation and OpenAPI generation, and notes that create schemas exclude auto-generated IDs.
Describe SQLAlchemy setup in FastAPI from database config to endpoint
Tests FastAPI dependency injection and SQLAlchemy session lifecycle. Good answers cover: engine with pooling, declarative models, a yield-based session dependency, and endpoint queries. Red flag: engine per request or global session shared everywhere.
Explain await's purpose, awaitable types, and event loop signaling
This tests whether you understand await as a yield point. A strong answer lists three awaitables (coroutines, Tasks, Futures), explains await yields control to the event loop until completion, and warns that a bare coroutine call does not run it.
What is the difference between async def and regular functions?
This tests async def call semantics. A strong answer contrasts regular functions, which execute immediately, with async def, which returns a coroutine object that does nothing until awaited or wrapped in create_task.
How do you include a router in your main FastAPI app?
Create APIRouter in another file, import it into main.py, then call app.include_router(router).
Describe a common FastAPI project structure and key directories
Main module holds the FastAPI app; domain files (users, items) expose APIRouters; dependencies in their own module; pyproject.toml as entrypoint.
What is APIRouter in FastAPI and why use it?
It tests your grasp of modular architecture in FastAPI. APIRouter splits routes into separate modules so you include them in the main app with prefixes, tags, and dependencies.
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.
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 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.
Define a FastAPI endpoint with path and query parameters
Tests if you know FastAPI infers parameter location from the route string. Good answer: route with {item_id}, signature item_id: int, q: str | None = None, noting any param not in the path becomes a query param.
How would you use a Pydantic response_model to enforce output structure?
Tests separation of internal models from API contracts. Define a Pydantic output model with only safe fields, set it as the endpoint response_model, and let FastAPI filter and validate.
How do you define a Pydantic model and use it in FastAPI?
Subclass BaseModel with name str and age int, then type-hint the parameter with the model.
What is the difference between a Pydantic default and Optional field?
Both forms are non-required; str = 'guest' rejects None, Optional[str] = None accepts it.
How does Pydantic handle extra JSON fields, and how to configure it?
This tests Pydantic's data filtering behavior and configuration. By default, Pydantic ignores extra fields silently. Set model_config = ConfigDict(extra='forbid' or 'allow') to change it. A red flag is claiming FastAPI 422s by default on unknown fields.
How do you define a Pydantic model for FastAPI request body validation?
Subclass BaseModel with id int, email str, full_name str|None; pass it as a route param so FastAPI validates JSON and returns 422s.
We are hiring for this. Every open role lists the topics its interview covers, so you can prepare for the real thing rather than guessing.
See open roles