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# Explaining the webapp2 Framework

The 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`. The `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.

## Hello, webapp2!

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 another look at our friendly greeting application:

    import webapp2

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

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

## What webapp2 Does

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.

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

## Next...

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.