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).
A webapp2
application has three parts:
RequestHandler
classes that process requests and build
responses.WSGIApplication
instance that routes incoming requests to handlers
based on the URL.WSGIApplication
using a CGI adaptor.Let’s rewrite our friendly greeting as a webapp2
application. Edit
helloworld/helloworld.py
and replace its contents with the following:
import webapp2
class MainPage(webapp2.RequestHandler):
def get(self):
self.response.headers['Content-Type'] = 'text/plain'
self.response.out.write('Hello, webapp2 World!')
application = webapp2.WSGIApplication([
('/', MainPage)
], debug=True)
Also edit app.yaml
and replace its contents with the following:
application: helloworld
version: 1
runtime: python27
api_version: 1
threadsafe: true
handlers:
- url: /.*
script: helloworld.application
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!”).
This code defines one request handler, MainPage
, mapped to the root URL
(/
). When webapp2
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. webapp2
sends a response based on
the final state of the MainPage
instance.
The application itself is represented by a webapp2.WSGIApplication
instance. The parameter debug=true
passed to its constructor tells
webapp2
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 code application.run()
runs the application in App Engine’s CGI
environment. It uses a function provided by App Engine that 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 webapp2
later in this tutorial. For more
information about webapp2
, see the webapp2 reference.
Frameworks make web application development easier, faster and less error prone. webapp2 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.