Python interview questions and answers

N+1 happens when you fetch a list, then access a related field per row, issuing one query plus N more.

F() references a field value in the database so updates happen in SQL, not in Python.

Q objects let you build complex WHERE clauses with | (OR), & (AND), and ~ (NOT).

Set db_index=True on a field, or declare indexes in Meta.indexes / unique_together / UniqueConstraint.

ForeignKey is many-to-one.

On the way in, Django runs middleware from top to bottom of the MIDDLEWARE list.

Legacy mixin-style middleware implemented process_request, process_view, process_exception, and process_response.

Define a class that takes get_response in __init__ and in __call__ runs pre-logic, calls get_response(request), then post-logic.

It should see the raw request early to apply SSL redirects and refuse bad hosts before other layers do work.

pre_save and pre_delete/post_delete wrap persistence.

By default Django stores receivers as weak references, so a local function can be garbage-collected and silently stop running.

Avoid them for core business rules that callers must understand

ModelSerializer maps model fields quickly and is enough for straightforward CRUD.

GenericAPIView plus mixins gives you get/post handlers and queryset/serializer_class.

DefaultRouter and SimpleRouter bind viewset actions to URL patterns including optional trailing slashes.

Authentication identifies the user (who is calling) and sets request.user and request.auth.

Pagination classes split list querysets into pages and add next/previous metadata.

Nest a serializer as a field, or use depth on ModelSerializer for read-only trees.

The broker (Redis, RabbitMQ) transports task messages to workers.

delay(*args, **kwargs) is a shortcut for apply_async.