On Google App Engine – First Impressions

Google’s App Engine offers an intriguing new rapid development opportunity. Long story short, Google is offering developers a chance to write apps against its App Engine API. Included are features such as a simple web app framework (appropriately called webapp), an object-based datastore (complete with GQL query language), Google authentication and support for memcache.

The webapp framework feels like a mix of Java servlet programming and MVC frameworks like Pylons or TurboGears. You write classes to handle requests and can optionally hook in a templating framework, such as mako. I’ve just started playing with App Engine, but put together a simple getting-started structure that I think is worth sharing. Walk through the Hello, World! sample to get an idea of what the api is like.

In my app.yaml file, I map all requests to main.py

...
handlers:
- url: /.*
  script: main.py

main.py simply initializes the web application.

from google.appengine.ext import webapp
from google.appengine.ext.webapp.util import run_wsgi_app

from homehandler import *

application = webapp.WSGIApplication(
                                     [('/', HomeHandler),
                                     debug=True)

def main():
  run_wsgi_app(application)

if __name__ == "__main__":
  main()

I've also created a base class for my handlers, AbstractHandler. This class provides a method for subclasses to wire in mako templates. The mako code is modified a little from this posting. I also create a property view_data that can be populated by subclasses and is passed to the templates in the render_template function.

from google.appengine.ext import webapp
from google.appengine.ext.webapp.util import run_wsgi_app

from mako.template import Template

import os

class AbstractHandler(webapp.RequestHandler):

	def __init__(self):

		self._view_data = {}

	@property
	def	view_data(self):

		return self._view_data

	def render_template(self, action):

		#assume default directory maps to home
		if len(self.request.path) > 1:
			controller = self.request.path
		else:
			controller = "\home"

		view_path = "views%s/%s.mako" % (controller, action)
		path = os.path.join(os.path.dirname(__file__), view_path)
		templ = Template(filename=path)
		self.response.out.write(templ.render(**self._view_data))

Finally, subclasses extend this class and implement the necessary verb methods.

from abstracthandler import *

class HomeHandler(AbstractHandler):

	def get(self):

		self.view_data["some_foo"] = "some foo"
		self.view_data["some_bar"] = "some bar"

		self.render_template("index")

The mako template (from the above mentioned blog) looks like this:

${some_foo} likes the ${some_bar}

Now I’ll be the first to admit, there’s nothing too exciting about my approach (and I’m still getting my feet wet with Python), but I think it’s a little better laid out than the samples you’ll find in the docs. I’m personally not a fan of having a giant web handler file with all handlers (same goes for models).