Quixote Programming Overview

This document explains how a Quixote application is structured. Be sure you have read the "Understanding the demo" section of demo.txt first -- this explains a lot of Quixote fundamentals.

There are three components to a Quixote application:

  1. A driver script, usually a CGI or FastCGI script. This is the interface between your web server (eg., Apache) and the bulk of your application code.

    The driver script is responsible for creating a Quixote publisher customized for your application and invoking its publishing loop.

  2. A configuration file. This file specifies various features of the Publisher class, such as how errors are handled, the paths of various log files, and various other things. Read through quixote/config.py for the full list of configuration settings.

    The most important configuration parameters are:


    e-mail address to which errors will be mailed


    file to which errors will be logged

    For development/debugging, you should also set DISPLAY_EXCEPTIONS true and SECURE_ERRORS false; the defaults are the reverse, to favour security over convenience.

  3. Finally, the bulk of the code will be in a Python package or module, called the root namespace. The Quixote publisher will be set up to start traversing at the root namespace.

Driver script

The driver script is the interface between your web server and Quixote's "publishing loop", which in turn is the gateway to your application code. Thus, there are two things that your Quixote driver script must do:

The publisher is responsible for translating URLs to Python objects and calling the appropriate function, method, or PTL template to retrieve the information and/or carry out the action requested by the URL.

The most important application-specific customization done by the driver script is to set the root namespace of your application. Broadly speaking, a namespace is any Python object with attributes. The most common namespaces are modules, packages, and class instances. The root namespace of a Quixote application is usually a Python package, although for a small application it could be a regular module.

The driver script can be very simple; for example, here is a trimmed-down version of demo.cgi, the driver script for the Quixote demo:

from quixote import enable_ptl, Publisher
app = Publisher("quixote.demo")

(Whether you install this as demo.cgi, demo.fcgi, demo.py, or whatever is up to you and your web server.)

That's almost the simplest possible case -- there's no application-specific configuration info apart from the root namespace. (The only way to make this simpler would be to remove the enable_ptl() and setup_logs() calls. The former would remove the ability to import PTL modules, which is at least half the fun with Quixote; the latter would disable Quixote's debug and error logging, which is very useful.)

Here's a slightly more elaborate example, for a hypothetical database of books:

from quixote import enable_ptl, Publisher
from quixote.config import Config

# Install the PTL import hook, so we can use PTL modules in this app

# Create a Publisher instance with the default configuration.
pub = Publisher('books')

# Read a config file to override some default values.

# Setup error and debug logging (do this after read_config(), so
# the settings in /www/conf/books.conf have an effect!).

# Enter the publishing main loop

The application code is kept in a package named simply 'books' in this example, so its name is provided as the root namespace when creating the Publisher instance.

The SessionPublisher class in quixote.publish can also be used; it provides session tracking. The changes required to use SessionPublisher would be:

from quixote.publish import SessionPublisher
pub = SessionPublisher(PACKAGE_NAME)

For details on session management, see session-mgmt.txt.

Getting the driver script to actually run is between you and your web server. See the web-server.txt document for help, especially with Apache (which is the only web server we currently know anything about).

Configuration file

In the books.cgi driver script, configuration information is read from a file by this line:


You should never edit the default values in quixote/config.py, because your edits will be lost if you upgrade to a newer Quixote version. You should certainly read it, though, to understand what all the configuration variables are.

The configuration file contains Python code, which is then evaluated using Python's built-in function execfile(). Since it's Python code, it's easy to set config variables:

ACCESS_LOG = "/www/log/access/books.log" 
DEBUG_LOG = "/www/log/books-debug.log"
ERROR_LOG = "/www/log/books-error.log"

You can also execute arbitrary Python code to figure out what the variables should be. The following example changes some settings to be more convenient for a developer when the WEB_MODE environment variable is the string DEVEL:

web_mode = os.environ["WEB_MODE"]
if web_mode == "DEVEL":
    RUN_ONCE = 1
elif web_mode in ("STAGING", "LIVE"):
    RUN_ONCE = 0
    raise RuntimeError, "unknown server mode: %s" % web_mode

At the MEMS Exchange, we use this flexibility to display tracebacks in DEVEL mode, to redirect generated e-mails to a staging address in STAGING mode, and to enable all features in LIVE mode.


Every Quixote application can have up to three log files, each of which is selected by a different configuration variable:

If you want logging to work, you must call setup_logs() on your Publisher object after creating it and reading any application-specific config file. (This only applies for CGI/FastCGI driver scripts, where you are responsible for creating the Publisher object. With mod_python under Apache, it's taken care of for you.)

Quixote writes one (rather long) line to the access log for each request it handles; we have split that line up here to make it easier to read: - 2001-10-15 09:48:43
  2504 "GET /catalog/ HTTP/1.1"
  200 'Opera/6.0 (Linux; U)' 0.10sec

This line consists of:

If no access log is configured (ie., ACCESS_LOG is None), then Quixote will not do any access logging.

The error log is used for two purposes:

If no error log is configured (with ERROR_LOG), then both types of messages will be written to the stderr supplied to Quixote for this request by your web server. At least for CGI/FastCGI scripts under Apache, this winds up in Apache's error log.

The debug log is where any application output to stdout goes. Thus, you can just sprinkle print statements into your application for debugging; if you have configured a debug log, those print statements will wind up there. If you don't configure a debug log, they go to the bit bucket (/dev/null on Unix, NUL on Windows).

Application code

Finally, we reach the most complicated part of a Quixote application. However, thanks to Quixote's design, everything you've ever learned about designing and writing Python code is applicable, so there are no new hoops to jump through. The only new language to learn is PTL, which is simply Python with a novel way of generating function return values -- see PTL.txt for details.

An application's code lives in a Python package that contains both .py and .ptl files. Complicated logic should be in .py files, while .ptl files, ideally, should contain only the logic needed to render your Web interface and basic objects as HTML. As long as your driver script calls enable_ptl(), you can import PTL modules (.ptl files) just as if they were Python modules.

Quixote's publisher will start at the root of this package, and will treat the rest of the URL as a path into the package's contents. Here are some examples, assuming that the URL_PREFIX is "/q", your web server is setup to rewrite /q requests as calls to (eg.) /www/cgi-bin/books.cgi, and the root package for your application is 'books':

http://.../q/         call         books._q_index()
http://.../q/other    call         books.other(), if books.other
                                   is callable (eg. a function or
http://.../q/other    redirect to  /q/other/, if books.other is a
                                   namespace (eg. a module or sub-package)
http://.../q/other/   call         books.other._q_index(), if books.other
                                   is a namespace

One of Quixote's design principles is "Be explicit." Therefore there's no complicated rule for remembering which functions in a module are public; you just have to list them all in the _q_exports variable, which should be a list of strings naming the public functions. You don't need to list the _q_index() function as being public; that's assumed. Eg. if foo() is a function to be exported (via Quixote to the web) from your application's namespace, you should have this somewhere in that namespace (ie. at module level in a module or __init__.py file):

_q_exports = ['foo']

When a function is callable from the web, it must expect a single parameter, which will be an instance of the HTTPRequest class. This object contains everything Quixote could discover about the current HTTP request -- CGI environment variables, form data, cookies, etc. When using SessionPublisher, request.session is a Session object for the user agent making the request.

The function should return a string; all PTL templates return a string automatically. request.response is an HTTPResponse instance, which has methods for setting the content-type of the function's output, generating an HTTP redirect, specifying arbitrary HTTP response headers, and other common tasks. (Actually, the request object also has a method for generating a redirect. It's usually better to use this -- ie. code request.redirect(...) because generating a redirect correctly requires knowledge of the request, and only the request object has that knowledge. request.response.redirect(...) only works if you supply an absolute URL, eg. "http://www.example.com/foo/bar".)


pydoc quixote.http_request
pydoc quixote.http_response

to view the documentation for the HTTPRequest and HTTPResponse classes, or consult the source code for all the gory details.

There are exactly two ways to affect the how Quixote traverses a URL to determine how to handle it: _q_access() and _q_getname().


If this function is present in a module, it will be called before attempting to traverse any further. It can look at the contents of request and decide if the traversal can continue; if not, it should raise quixote.errors.AccessError (or a subclass), and Quixote will return a 403 ("forbidden") HTTP status code. The return value is ignored if _q_access() doesn't raise an exception.

For example, in the MEMS Exchange code, we have some sets of pages that are only accessible to signed-in users of a certain type. The _q_access() function looks like this:

def _q_access (request):
    if request.session.user is None:
        raise NotLoggedInError("You must be signed in.")
    if not (request.session.user.is_MX() or
raise MXAccessError("You don't have access to the reports page.")

This is less error-prone than having to remember to add checks to every single public function.

_q_getname(request, name)

This function translates an arbitrary string into an object that we continue traversing. This is very handy; it lets you put user-space objects into your URL-space, eliminating the need for digging ID strings out of a query, or checking PATH_INFO after Quixote's done with it. But it is a compromise with security: it opens up the traversal algorithm to arbitrary names not listed in _q_exports. You should therefore be extremely paranoid about checking the value of name.

request is the request object, as it is everywhere else; name is a string containing the next chunk of the path. _q_getname() should return either a string (a complete document that will be returned to the client) or some object that can be traversed further. Returning a string is useful in simple cases, eg. if you want the /user/joe URI to show everything about user "joe" in your database, you would define a _q_getname() in the namespace that handles /user/ requests:

template _q_getname (request, name):
    if not request.session.user.is_admin():
        raise AccessError("permission denied")
    user = get_database().get_user(name)
    if user is None:
        raise TraversalError("no such user: %r" % name)
        "<h1>User %s</h1>\n" % html_quote(name)
        " <tr><th>real name</th><td>%s</td>\n" % user.real_name
        # ...

(This assumes that the namespace in question is a PTL module, not a Python module.)

To publish more complex objects, you'll want to use _q_getname()'s ability to return a new namespace that Quixote continues traversing. The usual way to do this is to return an instance of a class that implements the web front-end to your object. That class must have a _q_exports attribute, and it will almost certainly have a _q_index() method. It might also have _q_access() and _q_getname() (yes, _q_getname() calls can nest arbitrarily deeply).

For example, you might want /user/joe/ to show a summary, /user/joe/history to show a login history, /user/joe/prefs to be a page where joe can edit his personal preferences, etc. The _q_getname() function would then be

def _q_getname (request, name):
    return UserUI(request, name)

and the UserUI class, which implements the web interface to user objects, might look like

class UserUI:
    _q_exports = ['history', 'prefs']

    def __init__ (self, request, name):
        if not request.session.user.is_admin():
            raise AccessError("permission denied")
        self.user = get_database().get_user(name)
        if self.user is None:
            raise TraversalError("no such user: %r" % name)

    def _q_index (self, request):
        # ... generate summary page ...

    def history (self, request):
        # ... generate history page ...

    def prefs (self, request):
        # ... generate prefs-editing page ...

$Id: programming.txt,v 1.12 2002/10/01 21:57:27 gward Exp $