When comparing Nagare vs Klein, the Slant community recommends Nagare for most people. In the question“What are the best general-purpose Python web frameworks usable in production sites?” Nagare is ranked 14th while Klein is ranked 20th. The most important reason people chose Nagare is:
Nagare allows for simpler programming flow hiding client/server requests.
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Pros
Pro Continuation based web framework
Nagare allows for simpler programming flow hiding client/server requests.
Pro 100% Python web application development - no HTML, CSS or JS needed
Nagare is a mature web framework that saves you from manually creating routes. It basically is just a Python object graph.
Pro More concurrent requests, more interactivity
The fact that a Klein server is event-driven and non-blocking means that it can start handling a new request while previous requests are still open. This lets you serve more requests from a single process, meaning running multiple servers is now an option to be explored when your site makes it big, rather than a necessity for responsiveness under even modest loads.
Multiple requests per process also gives you flexibility to do things that would be impractical in WSGI-based alternatives like Flask or Bottle, such as keeping a connection to the browser open to send it chat messages or game updates in a Server Sent Event stream.
Cons
Con Stateful server that maintains both session and UI states server side
Con Uses old YUI2 JS framework
Nagare uses YUI2 internally for XHR requests. YUI2 has been deprecated since 2011.
Con Lacks database integration
Talking to database is a pretty common thing for a web application to do. Larger frameworks know this, and cover it in some detail by the end of the tutorial. In contrast, the Klein documentation is currently silent on this topic, leaving the issue of how to do database queries in a way that won't block your event-driven code entirely up to you.