Google App Engine - Go vs. Python recommendations?

I did a Python app for GAE recently and coming from C# I had no problems/was able to pick it up in a few days tinkering with the docs and playing with the SDK (I had some previous experience with python). Python is pretty intuitive, it's imperative and OO, might require a slight change in thinking from C# but nothing revolutionary - using the interactive interpreter will allow you to pick it up in a day IMO. Learning how to use the datastore and figuring out other GAE specific API's (blobstore, image, mail) will probably be more work (it's specifics like transactions/groups, consistency modes ...) but you can figure that stuff out as you go/when you need it in your app. One thing to note - recently announced pricing scheme makes Python a bad choice on GAE ATM because it cannot handle requests currently and this leads to one "instance" per request. This is also true for Go ATM (as far as I know). JVM OTOH can handle ~20 simultaneous requests per instance if I remember the mailing list conversations correctly. And you will be paying per instance/hour. This makes JVM the most practical choice if you plan on publishing your application in the near future. Also using JVM you could use Scala (a new/cool language) and a framework like Lift which should theoretically allow you to port your code/avoid lock-in (disclaimer : I say theoretically because I'm only starting out with Scala and have no practical experience with Lift). So I would recommend to look around the mailing list and see what other people are saying about the recent changes.

Personally I would prefer Python because is much, much more mature than Go. In the past, I learnt, the hard way, what are the risks of choosing a not-so-mature technology. Warning, I am biased to recommend Python because I like it but you should also be careful because I heard many complains about how much support does Google put on Python-App-Engine. People are asking for years to upgrade Python support and nobody hears them. These being said, probably Java would be a smarter choice, especially because you are used to C#.

This question is about as subjective as they come, but I'll bite anyway. Python is easier to learn, has a much larger development community, is mature, and has a lot of third-party libraries available for you to integrate into your application. It's a winner for sure. That said, Go is an extremely well-designed language. Far, far more so than Python. Go was specifically designed to allow you to catch most mistakes at compile time, while Python is almost legendary for its ability to mask your mistakes. Go code tends to be easier to maintain. Go is also dramatically more efficient than Python -- several times faster or even several orders of magnitude faster, depending on what you're doing. Both languages are very powerful and very fast to write code in -- you can accomplish a lot in a short amount of space. But Go is unfinished and still in a state of flux, with core APIs still changing. It has a comparatively small developer base, and very few "real world" usage examples. Nonetheless, even this early in its development, it's already shown to be a compelling alternative with a clear use case.

Go is still experimental on the GAE - so maybe not the best platform to learn a new language. Python is definitely a good choice. Considering a new language do not forget that by choosing Java you get the JVM which allows not only Java code but also oder languages like: Scala, Grails, Clojure, JRuby any many others.

Python is since long supported on GAE - lots of tested information, infrastructure, etc. Go is new to GAE. I would decide more on the language level. Coming from C#, choosing Python you go a bit more "high-level", whereas choosing Go goes more into the "low-level" direction. More low-level control, but also more concern about that. Choose Python if your interest is on application development, and Go if it's more about systems development. One important aspect for me would be that Go has actor-style concurrency built in, though I don't know how well supported this on GAE will be.