Instead of Flask: FastAPI

Why FastAPI is better

• Type hints do the validation. FastAPI uses Pydantic models to validate, serialise and deserialise data, so malformed requests are rejected without hand-written checks.
• OpenAPI documentation is generated automatically, with Swagger UI at /docs and ReDoc at /redoc.
• It is built on Starlette and ASGI, so async endpoints, WebSockets and dependency injection are part of the core. Flask is a WSGI framework.
• FastAPI is now the most used Python web framework in the JetBrains Python Developers Survey: 38% of respondents in 2024, up from 29% in 2023, ahead of Django at 35% and Flask at 34%.

When to keep Flask

Keep Flask for server-rendered HTML apps with Jinja templates, small internal tools, and existing apps that depend on Flask extensions. It is stable, well documented and maintained by the Pallets project.

Switching

Rewrite route by route. Flask view functions become FastAPI path operations with typed parameters, request parsing moves into Pydantic models, and Flask extensions are replaced with dependencies and middleware. You serve the app with an ASGI server such as Uvicorn. For a gradual move, FastAPI can mount the old Flask app through WSGI middleware.

Watch out

FastAPI's async model punishes blocking calls, because a synchronous database driver inside an async endpoint stalls other requests. It has no built-in ORM, admin or login system, so you assemble those yourself. Its development is led closely by its creator, so the project leans on one person's direction.

Also worth a look

• Litestar An ASGI framework in the same typed style as FastAPI, run by a team rather than one lead; pick it if you want more built in, such as a data layer and DTOs.

More newer, better web frameworks

• Express →, use Hono
• Gatsby →, use Astro