FastAPI is an async-first framework built for APIs, while Flask is a minimal synchronous framework that leaves most choices to you. FastAPI runs on the Asynchronous Server Gateway Interface (ASGI), the async standard between Python servers and apps. It reads your type hints to validate requests, serialize responses and generate API docs. Flask runs on the older Web Server Gateway Interface (WSGI), so each request holds a worker for its whole life, even in an async view.
| FastAPI | Flask | |
|---|---|---|
| Server interface | ASGI, async by default | WSGI, sync by default |
| Validation | Built in, from type hints through Pydantic | Manual or an extension |
| API docs | Generated automatically | Needs an extension |
| Serialization | Pydantic models and response models | A returned dict or list becomes JSON, complex types need a library |
| Style | Modern Python, with type hints declaring inputs and outputs | More traditional approach, reading inputs from the global request object |
| Performance | Better for many concurrent I/O-bound requests | Lower, since each request ties up a worker |
| Getting started | Needs type hints and some async knowledge | Simpler for basic apps |
Pick FastAPI for a new API that needs high concurrency, request validation and documentation without extra packages. Pick Flask when you want flexibility, simple server-rendered pages with its built-in Jinja templates, or its large set of mature extensions.
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