[treyd]

That's correct. Flask has a global request context object, so by design it can only safely handle a single request at a time per Python interpreter. If you want to parallelize multiple Flask servers, you spin up multiple interpreters.

Web services in Python that want to handle multiple comcurrent requests in the same interpreter should be using a web framework that is designed around that expectation and don't use a global request context object, such as FastAPI.

[formerly_proven]

No, Flask has global objects wrapping state objects in contextvars.

[nine_k]

This is even more funny, because now you need to switch the global reference according to the context. With GIL, it is easy; without it,...

[formerly_proven]

You misunderstand. The "request" or "g" objects in Flask are proxies which access the actual objects through contextvars, which are effectively thread-local storage with some extra sugar. The context stack of a contextvar is already within the TLS and therefore always bound to a specific thread.