Django & FastAPI interview questions

ORM behaviour and N+1, middleware, dependency injection, async endpoints, and validation.

3 questions in Python.

Python across backend, data and automationA 20-minute read on how the topic fits together, before the questions.Python fundamentals60 short answers on one page, for revising rather than studying.

medium1

mediumConceptScenario

You cached something in a module-level dictionary. It works on your laptop and behaves oddly in production. What is different?

Production runs several worker processes, and each has its own interpreter, its own imported modules and therefore its own copy of that dictionary. Requests land on whichever worker is free, so a write in one is invisible to the others and every restart discards the lot. Use this wsgi asgi answer to show the decision, trade-off, and evidence rather than a memorised definition.

5 minmid, senior

hard2

hardScenarioConcept

A list endpoint that returns fifty orders issues more than a hundred queries, and rewriting it as an async endpoint made it slower. Explain both.

An N+1 happens when the ORM follows a relation lazily once per row; select_related fixes forward foreign keys with a SQL join, prefetch_related issues one extra query per relation and joins in Python. An async endpoint gets slower when it contains a blocking call, because that stalls the whole event loop.

5 minmid, senior, staff

hardScenarioConceptDesign

A worker picks up the job you queued and cannot find the row it was told about. What went wrong?

The job was enqueued inside an open transaction, so the row was invisible to the worker's connection until commit, and a fast worker won the race. Register the side effect with transaction.on_commit so it fires after the outermost atomic block commits, and never if it rolls back.

5 minmid, senior, staff