I'd say the most powerful part of FastAPI is that it integrates everything in a relatively opinionated way, that you would spend a lot of time setting up Flask plugins/extensions for to get an equivalent feature set.

I'm not sure I'd say it's based on OpenAPI(?), but it does automatically generate OpenAPI based docs from your Python code. There's no intermediate step, the '.../docs' route just exists at runtime.

I'm working on it 2 days a week beside writing the book.

The next major milestone is receiving emails in the inbox, but I think it will take 2-4 months to have it working.