Hello, webapp2!

Before we get started, confirm that your app.yaml file specifies the Python 2.7 runtime. If you auto-generated app.yaml via the Launcher, you'll see something along these lines:

application: helloworld
version: 1
runtime: python27
api_version: 1
threadsafe: yes

handlers:
- url: /favicon\.ico
  static_files: favicon.ico
  upload: favicon\.ico

- url: .*
  script: main.app

libraries:
- name: webapp2
  version: "2.5.1"

Now, let's take a closer look at the Python code. A webapp2 application has two 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

Let's take a look at the Greeting application you generated in the Introduction to App Engine class:

import webapp2

class MainPage(webapp2.RequestHandler):
  def get(self):
    self.response.headers['Content-Type'] = 'text/plain'
    self.response.out.write('Hello, webapp2 World!')

app = webapp2.WSGIApplication([('/', MainPage)], debug=True)

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 and call methods on self.response to prepare the response, then exits. webapp2 sends a response based on the final state of the response object 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.

We'll use a few more features of webapp2 later in this course. For more information, see the webapp2 documentation .

Summary and Review

Frameworks make web application development easier, faster, and less error-prone. webapp2 is just one of many such frameworks available for Python, which you can use to add features to your application.

In the next lesson , we'll start with a look at the Users service.