CherryPy / sphinx / source / concepts / basics.rst

Robert Brewer 6e177f1 

Jason R. Coombs 14b1021 
Robert Brewer 6e177f1 

Jason R. Coombs 14b1021 
Robert Brewer 6e177f1 

Jason R. Coombs 14b1021 
Robert Brewer 6e177f1 
Jason R. Coombs 14b1021 
Robert Brewer 6e177f1 

Jason R. Coombs 14b1021 

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!" = True


We assume that you already have :doc:`installed </intro/install>` 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
 * ``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:

 * :class:`cherrypy.engine <cherrypy.process.wspbus.Bus>`
   controls process startup, shutdown, and other events, including your own
   Plugins. See :doc:`/concepts/engine`.
 * :class:`cherrypy.server <cherrypy._cpserver.Server>` configures and controls
   the HTTP server.
 * :class:`cherrypy.request <cherrypy._cprequest.Request>` contains all
   the information that comes with the HTTP request, after it is parsed and
   analyzed by CherryPy.
 * :attr:`cherrypy.request.headers <cherrypy.lib.httputil.HeaderMap>`
   contains a mapping with the header options that were sent as part of
   the request.
 * :class:`cherrypy.session <cherrypy.lib.sessions.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.
 * :class:`cherrypy.response <cherrypy._cprequest.Response>` contains the
   data that is used to build the HTTP response.
 * :attr:`cherrypy.response.headers <cherrypy.lib.httputil.HeaderMap>`
   contains a mapping with the header options that will be returned by the
   server, before the contents get sent.
 * :attr:`cherrypy.response.body <cherrypy._cprequest.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::

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

Or to set headers on the response::

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