Using the webapp Framework

The CGI standard is simple, but it would be cumbersome to write all of the code that uses it by hand. Web application frameworks handle these details for you, so you can focus your development efforts on your application's features. Google App Engine supports any framework written in pure Python that speaks CGI (and any WSGI-compliant framework using a CGI adaptor), including Django, CherryPy, Pylons, and web.py. You can bundle a framework of your choosing with your application code by copying its code into your application directory.

App Engine includes a simple web application framework of its own, called webapp. The webapp framework is already installed in the App Engine environment and in the SDK, so you do not need to bundle it with your application code to use it. We will use webapp for the rest of this tutorial.

Hello, webapp!

A webapp application has three parts:

• one or more RequestHandler classes that process requests and build responses
• a WSGIApplication instance that routes incoming requests to handlers based on the URL
• a main routine that runs the WSGIApplication using a CGI adaptor

Let's rewrite our friendly greeting as a webapp application. Edit helloworld/helloworld.py and replace its contents with the following:

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

class MainPage(webapp.RequestHandler):
  def get(self):
    self.response.headers['Content-Type'] = 'text/plain'
    self.response.out.write('Hello, webapp World!')

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

def main():
  run_wsgi_app(application)

if __name__ == "__main__":
  main()

Reload http://localhost:8080/ in your browser to see the new version in action. (If you stopped your web server, restart it by running the command described in "Hello, World!".)

What webapp Does

The webapp module is in the google.appengine.ext package. This module is provided in the SDK, as well as in the production runtime environment.

This code defines one request handler, MainPage, mapped to the root URL (/). When webapp receives an HTTP GET request to the URL /, it instantiates the MainPage class and calls the instance's get method. Inside the method, information about the request is available using self.request. Typically, the method sets properties on self.response to prepare the response, then exits. webapp sends a response based on the final state of the MainPage instance.

The application itself is represented by a webapp.WSGIApplication instance. The parameter debug=true passed to its constructor tells webapp to print stack traces to the browser output if a handler encounters an error or raises an uncaught exception. You may wish to remove this option from the final version of your application.

The function run_wsgi_app() takes a WSGIApplication instance (or another WSGI-compatible application object) and runs it in App Engine's CGI environment. run_wsgi_app() is similar to the WSGI-to-CGI adaptor provided by the wsgiref module in the Python standard library, but includes a few additional features. For example, it can automatically detect whether the application is running in the development server or on App Engine, and display errors in the browser if it is running on the development server.

We'll use a few more features of webapp later in this tutorial. For more information about webapp, see the webapp reference.

Next...

Frameworks make web application development easier, faster and less error prone. webapp is just one of many such frameworks available for Python. Now that we're using a framework, let's add some features.

Continue to Using the Users Service.