The Web Server Gateway Interface (WSGI) 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 WSGI, including Django, CherryPy, Pylons, web.py, and web2py. 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, called
webapp2 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
webapp2 for the rest of this tutorial.
webapp2 application has two parts:
- one or more
RequestHandlerclasses that process requests and build responses
WSGIApplicationinstance that routes incoming requests to handlers based on the URL
Let's take another look at our friendly greeting application:
What webapp2 Does
This code defines one request handler,
MainPage, mapped to the root URL (
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
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. This option should be removed before deploying the final version of your application, otherwise you will inadvertently expose the internals of your application.