# Your first CherryPy application

The standard 'Hello world!' application takes less than 10 lines of code when written using CherryPy:

import cherrypy

class HelloWorld:
def index(self):
return "Hello world!"
index.exposed = True

cherrypy.quickstart(HelloWorld())


We assume that you already have :doc:installed </intro/install> CherryPy. Copy the file above and save it locally as hello.py, then start the application at the command prompt:

\$ python hello.py


Direct your favorite web browser to http://localhost:8080 and you should see Hello world! printed there.

## How does it work?

Let's take a look at hello.py:

• The import cherrypy statement imports the main CherryPy module. This is all that is required to have CherryPy working. Feel free to "import cherrypy" in an interactive session and see what's available! help(cherrypy) is also quite useful.
• We declare a class named HelloWorld. An instance of this class is the object that will be published by CherryPy. It contains a single method, named index, which will get called when the root URL for the site is requested (for example, http://localhost/). This method returns the contents of the Web page; in this case, the 'Hello World!' string. Note that you don't have to subclass any framework-provided classes; in fact, you don't even have to use classes at all! But let's start with them for now.
• The index.exposed = True is a necessary step to tell CherryPy that the index() method will be exposed. Only exposed methods can be called to answer a request. This feature allows the user to select which methods of an object will be accessible via the Web; non-exposed methods can't be accessed.
• cherrypy.quickstart(HelloWorld()) mounts an instance of the HelloWorld class, and starts the embedded webserver. It runs until explicitly interrupted, either with Ctrl-C or via a suitable signal (a simple kill on Unix will do it).

When the application is executed, the CherryPy server is started with the default configuration. It will listen on localhost at port 8080. These defaults can be overridden by using a configuration file or dictionary (more on this later).

Finally, the web server receives the request for the URL http://localhost:8080. It searches for the best method to handle the request, starting from the HelloWorld instance. In this particular case, the root of the site is automatically mapped to the index() method (similar to the index.html that is the standard page for conventional Web servers). The HelloWorld class defines an index() method and exposes it. CherryPy calls HelloWorld().index(), and the result of the call is sent back to the browser as the contents of the index page for the website. All the dispatching and HTTP-processing work is done automatically; the application programmer only needs to provide the desired content as the return value of the index method.

## CherryPy structure

Most of the features of CherryPy are available through the :mod:cherrypy module. It contains several members:

## CherryPy Response

The cherrypy.response object is available to affect aspects of the response to a request. Like the request, the response object is a thread-local, meaning although it appears to be a global variable, its value is specific to the current thread, and thus the current request.

One may store arbitrary data in the response object.

CherryPy exposes the request headers (as sent from the client), and response headers (to be returned in the response) in the headers attribute of cherrypy.request and cherrypy.response.

For example, to find out what "host" to which the client intended to connect:

@cherrypy.expose
def index(self):

@cherrypy.expose