I wonder if it's possible to attach a middleware/function for a specific route like express js. http://expressjs.com/en/guide/using-middleware.html#middleware.application
In my understanding, it seems that flask(WSGI) middleware applies to all request.
The equivalent pattern is a route decorator. Flask has some docs about it. It is pretty flexible, you can validate anything in HTTP request and change the response based on that.
For example, Flask-Login has the @login_required decorator to add a user authentication step before the route is called.