Flask vs webapp2 for Google App Engine
I think google app engine officially supports flask framework. There is a sample code and tutorial here -> https://console.developers.google.com/start/appengine?_ga=1.36257892.596387946.1427891855
For me the decision for webapp2 was easy when I discovered that flask is not an object-oriented framework (from the beginning), while webapp2 is a pure object oriented framework. webapp2 uses Method Based Dispatching as standard for all RequestHandlers (as flask documentation calls it and implements it since V0.7 in MethodViews). While in flask MethodViews are an add-on it is a core design principle for webapp2. So your software design will look different using both frameworks. Both frameworks use nowadays jinja2 templates and are fairly feature identical.
I prefer to add security checks to a base-class RequestHandler and inherit from it. This is also good for utility functions, etc. As you can see for example in link [3] you can override methods to prevent dispatching a request.
If you are an OO-person, or if you need to design a REST-server, I would recommend webapp2 for you. If you prefer simple functions with decorators as handlers for multiple request-types, or you are uncomfortable with OO-inheritance then choose flask. I think both frameworks avoid the complexity and dependencies of much bigger frameworks like pyramid.
- http://flask.pocoo.org/docs/0.10/views/#method-based-dispatching
- https://webapp-improved.appspot.com/guide/handlers.html
- https://webapp-improved.appspot.com/guide/handlers.html#overriding-dispatch
Disclaimer: I'm the author of tipfy and webapp2.
A big advantage of sticking with webapp (or its natural evolution, webapp2) is that you don't have to create your own versions for existing SDK handlers for your framework of your choice.
For example, deferred uses a webapp handler. To use it in a pure Flask view, using werkzeug.Request and werkzeug.Response, you'll need to implement deferred for it (like I did here for tipfy).
The same happens for other handlers: blobstore (Werkzeug still doesn't support range requests, so you'll need to use WebOb even if you create your own handler -- see tipfy.appengine.blobstore), mail, XMPP and so on, or others that are included in the SDK in the future.
And the same happens for libraries created with App Engine in mind, like ProtoRPC, which is based on webapp and would need a port or adapter to work with other frameworks, if you don't want to mix webapp and your-framework-of-choice handlers in the same app.
So, even if you choose a different framework, you'll end a) using webapp in some special cases or b) having to create and maintain your versions for specific SDK handlers or features, if you'll use them.
I much prefer Werkzeug over WebOb, but after over one year porting and maintaining versions of the SDK handlers that work natively with tipfy, I realized that this is a lost cause -- to support GAE for the long term, best is to stay close to webapp/WebOb. It makes support for SDK libraries a breeze, maintenance becomes a lot easier, it is more future-proof as new libraries and SDK features will work out of the box and there's the benefit of a large community working around the same App Engine tools.
A specific webapp2 defense is summarized here. Add to those that webapp2 can be used outside of App Engine and is easy to be customized to look like popular micro-frameworks and you have a good set of compelling reasons to go for it. Also, webapp2 has a big chance to be included in a future SDK release (this is extra-official, don't quote me :-) which will push it forward and bring new developers and contributions.
That said, I'm a big fan of Werkzeug and the Pocoo guys and borrowed a lot from Flask and others (web.py, Tornado), but -- and, you know, I'm biased -- the above webapp2 benefits should be taken into account.
Your question is extremely broad, but there appears to be no big problems using Flask on Google App Engine.
This mailing list thread links to several templates:
http://flask.pocoo.org/mailinglist/archive/2011/3/27/google-app-engine/#4f95bab1627a24922c60ad1d0a0a8e44
And here is a tutorial specific to the Flask / App Engine combination:
http://www.franciscosouza.com/2010/08/flying-with-flask-on-google-app-engine/
Also, see App Engine - Difficulty Accessing Twitter Data - Flask, Flask message flashing fails across redirects, and How do I manage third-party Python libraries with Google App Engine? (virtualenv? pip?) for issues people have had with Flask and Google App Engine.