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I believe Flask 2.0 optionally supports it whereas FastAPI is async only. I could be wrong though.

Edit: Thanks for the correction, my point is moot and irrelevant.

FastAPI happily supports both options in a gradual manner. Just start out sync and convert to async if you want on a per-route basis.

That's not exactly accurate. When working with sync views FastAPI dumps them into a threadpool. It basically converts them to async on the fly. To quote the docs-

> When you declare a path operation function with normal def instead of async def, it is run in an external threadpool that is then awaited, instead of being called directly (as it would block the server).

FastAPI allows you to define non-async views.

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