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Python / FastAPI Basics Interview Questions

What is FastAPI and what are its key advantages over Flask or Django REST Framework?

FastAPI is a modern, high-performance Python web framework for building APIs, built on top of Starlette (for async web handling) and Pydantic (for data validation). It was created by Sebastián Ramírez and released in 2018.

FastAPI vs Flask vs DRF
FeatureFastAPIFlaskDjango REST Framework
PerformanceVery high (async, Starlette)Moderate (sync-first)Moderate (sync-first)
Type hintsFirst-class — drives validation & docsOptional, no built-in useLimited
Auto docsSwagger UI + ReDoc built-inManual setup requiredBrowsable API only
ValidationPydantic — automatic, deepManual or extensionsSerializers — verbose
Async supportNative (async def)Limited (Flask 2+)Limited
Learning curveLow-moderateLowHigh (Django ecosystem)

Key selling points:

  • Automatic interactive API documentation (Swagger UI at /docs, ReDoc at /redoc)
  • Runtime data validation and serialisation via Pydantic with zero extra code
  • Full async/await support enabling high concurrency
  • Editor auto-completion everywhere because the entire framework is built around type hints
  • One of the fastest Python frameworks available — comparable to NodeJS and Go in benchmarks
What two libraries does FastAPI build on top of?
Where does FastAPI's automatic interactive documentation appear by default?

More Related questions...

What is FastAPI and what are its key advantages over Flask or Django REST Framework? How do you create and run a minimal FastAPI application? What is the difference between path parameters and query parameters in FastAPI? How do you receive and validate a JSON request body in FastAPI? How do you use Pydantic models for data validation and what validation features does FastAPI support? What is the response_model parameter in FastAPI and why should you use it? How do you add validation constraints to path and query parameters using Path() and Query()? How do you control HTTP status codes and return custom responses in FastAPI? What is FastAPI's dependency injection system and how do you use it? How do you organise a FastAPI application with multiple routers (APIRouter)? What is middleware in FastAPI and how do you add custom middleware? When should you use async def vs def for route handlers in FastAPI? What are BackgroundTasks in FastAPI and when should you use them? How do you implement OAuth2 password flow with JWT tokens in FastAPI? How do you implement role-based access control (RBAC) using FastAPI dependencies? How do you integrate an async SQLAlchemy database with FastAPI? How do you manage database schema migrations in a FastAPI project with Alembic? How do you write tests for a FastAPI application using pytest and TestClient? How do you create custom exception handlers in FastAPI? How do you handle form data and file uploads in FastAPI? How do you manage environment variables and settings in FastAPI with Pydantic Settings? How do you run startup and shutdown logic in FastAPI using lifespan? How do you implement WebSocket endpoints in FastAPI? How do you containerise and deploy a FastAPI application with Docker? What are the key production deployment considerations for a FastAPI application? What are the key differences between Pydantic v1 and v2, and how does FastAPI use Pydantic v2? How do you add caching to FastAPI endpoints to improve performance? How do you customise the OpenAPI documentation in FastAPI? How do you integrate FastAPI with Celery for reliable background task processing? How do you measure and improve the performance of a FastAPI application? How do you use class-based dependencies and sub-dependencies in FastAPI? How do you test async FastAPI endpoints and async dependencies? How do you stream large responses in FastAPI using StreamingResponse? How do you add GraphQL support to a FastAPI application with Strawberry? How does FastAPI handle validation errors and how can you customise the error response format? What is the scope of a FastAPI dependency, and how do you share state across requests? How do you read HTTP headers and cookies in FastAPI? What are the most important FastAPI best practices for a production-ready API?
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