Your first CherryPy application

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

import cherrypy

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


We assume that you already have installed CherryPy. Copy the file above and save it locally as, then start the application at the command prompt:

$ python

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

  • 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 = 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. Another way to expose a method is to use the decorator cherrypy.expose().
  • 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 cherrypy module. It contains several members:

  • cherrypy.engine controls process startup, shutdown, and other events, including your own Plugins. See The CherryPy Engine.
  • cherrypy.server configures and controls the HTTP server.
  • cherrypy.request contains all the information that comes with the HTTP request, after it is parsed and analyzed by CherryPy.
  • cherrypy.request.headers contains a mapping with the header options that were sent as part of the request.
  • cherrypy.session is a special mapping that is automatically generated and encoded by CherryPy; it can be used to store session-data in a persistent cookie. For it to work you have to enable the session functionality by setting ‘tools.session.on’ to True in your config.
  • cherrypy.response contains the data that is used to build the HTTP response.
  • cherrypy.response.headers contains a mapping with the header options that will be returned by the server, before the contents get sent.
  • cherrypy.response.body contains the actual contents of the webpage that will be sent as a response.

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.

HTTP Headers

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:

import cherrypy

class HelloWorld(object):
    def index(self):
        host = cherrypy.request.headers['Host']
        return "You have successfully reached " + host


Or to set headers on the response:

import cherrypy

class HelloWorld(object):
    def _get_jpeg_data(self):
        """This method should return the jpeg data"""
        return ""

    def index(self):
        cherrypy.response.headers['Content-Type'] = 'application/jpeg'
        return self._get_jpeg_data()