tiangolo.fastapi/docs/en/docs/advanced/wsgi.md

Including WSGI - Flask, Django, others

You can mount WSGI applications as you saw with Sub Applications - Mounts{.internal-link target=_blank}, Behind a Proxy{.internal-link target=_blank}.

For that, you can use the WSGIMiddleware and use it to wrap your WSGI application, for example, Flask, Django, etc.

Using WSGIMiddleware

You need to import WSGIMiddleware.

Then wrap the WSGI (e.g. Flask) app with the middleware.

And then mount that under a path.

{* ../../docs_src/wsgi/tutorial001_py39.py hl[2:3,3] *}

Check it

Now, every request under the path /v1/ will be handled by the Flask application.

And the rest will be handled by FastAPI.

If you run it and go to http://localhost:8000/v1/ you will see the response from Flask:

Hello, World from Flask!

And if you go to http://localhost:8000/v2 you will see the response from FastAPI:

{
    "message": "Hello World"
}