Google's App Engine is a new application platform from Google that allows you to run custom Python code on Google's infrastructure. From their website: Several websites (including Google) make the claim that App Engine is compatible with any WSGI framework, even mentioning CherryPy by name. Unfortunately, they don't give any examples using CherryPy, and it turns out that it doesn't quite work "out of the box". Fortunately it wasn't too tricky to get it working, so I documented the steps here.

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Prerequisites

• Install Python 2.5 if you don't already have it.
• Install the AppEngine SDK
  You do not need a Google App Engine account to begin developing and running your own apps locally.
  
  

Create your application environment

Create a new folder where your application will live. For the example here, I'm using c:\hello

As noted above, CherryPy doesn't quite work "out of the box". I was only able to get it work with the latest beta version, but even then I had to patch it a bit. I'm sure there is a more elegant way to accomplish this than what I did (simply removing the offending code), but this works for now.

For ease of use I've provided a prepatched version of CherryPy:

• Download CherryPy patched for AppEngine
• Unpack the above file into c:\hello so that you end up with c:\hello\cherrypy.
  
  

If you'd rather do the patching yourself, here is the patch

Add your application code

Now to get down to business:

• Create hello.py in your application folder (i.e. c:\hello):
  hello.py

  """
  Recipe for running a CherryPy-based server under the Google AppEngine.
  There is actually nothing AppEngine-specific here - the key is setting up
  the environment correctly by following the other steps.
  """
  import cherrypy
  import wsgiref.handlers
  
  # application goes here ...
  class Root:
      def index(self):
          return "Hello, CherryPy! version=",cherrypy.__version__
          
      index.exposed = True
  
  #---------------------------------------------------------------------------
  # Start the server under Google AppEngine
  #---------------------------------------------------------------------------
  app = cherrypy.tree.mount(Root(), "/")
  wsgiref.handlers.CGIHandler().run(app)
• Create app.yaml in the same folder:
  app.yaml

  application: hello
  version: 1
  runtime: python
  api_version: 1
  
  handlers:
  - url: /.*
    script: hello.py
  
  
  

Running your application

Assuming you placed the AppEngine installation folder in your PATH (i.e. c:\Program Files\Google\google_appengine), you can run your server like this:

The first time you run an App Engine application, it will prompt you about whether it should automatically check for SDK updates. I answered "yes", but I don't think that's required.

You should now be able to view your application through http://127.0.0.1:8080. When you are finished, you can stop the application with Ctrl-C in the console window.



I hope this brief tutorial has saved you some pain. Have fun!

Update (Apr 19, 2008): A kind Googler was nice enough to set me up with an App Engine account, so I've created a tutorial site at http://boodebr.appspot.com