What is FastAPI, and what does it provide for a backend API?

Defines FastAPI as an ASGI web framework and explains how types drive validation, dependency resolution, responses, and OpenAPI.

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FastAPI is a Python web framework for building APIs on the ASGI interface. It uses Starlette for web and ASGI behavior and Pydantic for typed validation and serialization. From ordinary Python function signatures and type annotations, it can extract request data, validate it, resolve dependencies, shape responses, and generate an OpenAPI document. It removes a lot of transport plumbing, but application rules, persistence, authorization decisions, and operational design still belong to the application.

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The useful interview answer explains both what FastAPI is and which backend boundaries it handles.
FastAPI is a Python web framework for building APIs on the ASGI interface. It uses Starlette for web and ASGI behavior and Pydantic for typed validation and serialization. From ordinary Python function signatures and type annotations, it can extract request data, validate it, resolve dependencies, shape responses, and generate an OpenAPI document. It removes a lot of transport plumbing, but application rules, persistence, authorization decisions, and operational design still belong to the application.
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I treat FastAPI as the HTTP adapter around use cases rather than the architecture itself. A path operation declares the transport contract, dependencies obtain request-scoped collaborators or context, and a response model constrains the public output. That produces useful documentation and consistent boundary validation with little duplication. The trade-off is that annotation-driven behavior can become implicit, so parameter sources, dependency lifetimes, and response rules still need deliberate design and tests.

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Calling FastAPI an async function library rather than a web framework.
Claiming type hints enforce every business rule automatically.
Describing generated documentation as the API contract by itself.
Assuming use of FastAPI makes blocking application code non-blocking.

What is FastAPI, and what does it provide for a backend API?

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