WebKit User's Guide

Version 0.8, Webware for Python

Contents

Synopsis

WebKit provides Python classes for generating dynamic content from a web-based, server-side application. It is a significantly more powerful alternative to CGI scripts for application-oriented development.

Feedback

You can e-mail webware-discuss@lists.sourceforge.net to give feedback, discuss features, and get help using WebKit. If you have a bug to report, use the bug tracker.

Introduction

Python 9

A paper titled "Introduction to Webware" was accepted for Python 9, which ran the week of March 4th, 2001. @@ ib: where are these now?

Overview

The core concepts of the WebKit are the Application, Servlet, Request, Response and Transaction, for which there are one or more Python classes.

The application resides on the server-side and manages incoming requests in order to deliver them to servlets which then produce responses that get sent back to the client. A transaction is a simple container object that holds references to all of these objects and is accessible to all of them.

Content is normally served in HTML or XML format over an HTTP connection. However, applications can provide other forms of content and the framework is designed to allow new classes for supporting protocols other than HTTP.

In order to connect the web server and the application server a small program called an adapter is used. It bundles a web browser request and sends it to the application server, which then processes it and sends the response back to the adapter which then outputs the results for use by the web server. Adapters come in various flavors including CGI, FastCGI and Apache mod. See the Install Guide for more information.

At a more detailed level, the process looks like this:

  • At some point, someone has configured and run both a web server (such as Apache) and the WebKit app server (WebKit/AppServer).
  • A user requests a web page by typing a URL or submitting a form.
  • The user's browser sends the request to the remote web server.
  • The web server invokes the adapter.
  • The adapter simply collects information about the request and sends it to the WebKit app server which is ready and waiting.
  • The app server asks the Application object to dispatch the raw request.
  • The application instantiates an HTTPRequest object and asks the appropriate Servlet (as determined by examining the URL) to process it.
  • The servlet generates content into a given HTTPResponse object, whose content is then sent back by the app server to the adapter.
  • The adapter sends the content through the web server and ultimately to the user's web browser.

For a more detailed guide, see Anatomy of a Webware Transaction.

Compared to CGI applications

The alternative to a server-side application is a set of CGI scripts. However, a CGI script must always be launched from scratch and many common tasks will be performed repeatedly for each request. For example, loading libraries, opening database connections, reading configuration files, etc.

With the long-running server-side application, the majority of these tasks can be done once at launch time and important results can be easily cached. This makes the application significantly more efficient.

Of course, CGIs can still be appropriate for "one shot" deals or simple applications. Webware includes a CGI Wrapper if you'd like to encapsulate your CGI scripts with robust error handling, e-mail notifications, etc. @@ ib: is this still really supported?

Errors / Uncaught Exceptions

One of the conveniences provided by WebKit is the handling of uncaught exceptions. The response to an uncaught exception is:

Here is a sample error page.

Archived error messages can be browsed through the administration page.

Error handling behavior can be configured as described in Configuration.

Configuration

There are several configuration parameters through which you can alter how WebKit behaves. They are described below, including their default values. Note that you can override the defaults by placing config files in the Configs/ directory. A config file simply contains a Python dictionary containing the items you wish to override. For example:

{
    'SessionStore': 'Memory',
    'ShowDebugInfoOnErrors': 1
}

See the Configuration Guide for more information on settings.

Administration

WebKit has a built-in administration page that you can access via the Admin context. You can see a list of all contexts in the sidebar of any Example or Admin page.

The admin pages allows you to view WebKit's configuration, logs, and servlet cache, and perform actions such as clearing the cache, reloading selected modules and shutting down the app server.

More sensitive pages that give control over the app server require a user name and password, the username is admin, and you'll set the password when you run install.py. You can change the password in WebKit/Configs/Application.config.

The adminstration scripts provide further examples of writing pages with WebKit, so you may wish to examine their source in WebKit/Admin/.

Debugging

print

The most common technique is the infamous print statement. The results of print statements go to the console where the WebKit application server was started (not to the HTML page as would happen with CGI). Prefixing the debugging output with a special tag (such as >>) is useful because it stands out on the console and you can search for the tag in source code to remove the print statements after they are no longer useful. For example:

print '>> fields =', self.request().fields()

Raising Exceptions

Uncaught expections are trapped at the application level where a useful error page is saved with information such as the traceback, environment, fields, etc. You can configure the application to automatically e-mail you this information. Here is an example error page.

When an application isn't behaving correctly, raising an exception can be useful because of the additional information that comes with it. Exceptions can be coupled with messages, thereby turning them into more powerful versions of the print statement. For example:

raise Exception, 'self = %s' % self

Restarting the Server

When a servlet's source code changes, it is reloaded. However, ancestor classes of servlets and library modules are not. You may wish to use the auto-reloading feature to mitigate this problem.

In any case, when having problems, consider restarting the app server.

Another option is to use the AppControl page of the Admin context to clear the servlet instance and class cache.

Assertions

Assertions are used to ensure that the internal conditions of the application are as expected. An assertion is equivalent to an if statement coupled with an exception. For example:

assert shoppingCart.total()>=0.0, \
    'shopping cart total is %0.2f' % shoppingCart.total()

HTML Validation

You can validate the HTML in your pages using the Web Designer Group's HTML Validator. It is available as a RPM package, in Debian as wdg-html-validator and you can also install the source.

To enable the validation, you have to override .writeBodyParts() in your SitePage, as:

def writeBodyParts(self):
    Page.writeBodyParts()
    self.validateHTML()

If your pages contain invalid HTML, a message will be appended to the page.

Naming Conventions

Cookies and form values that are named with surrounding underscores (such as _sid_ and _action_) are generally reserved by WebKit and various plugins and extensions for their own internal purposes. If you refrain from using surrounding underscores in your own names, then [a] you won't accidentally clobber an already existing internal name and [b] when new names are introduced by future versions of WebKit, they won't break your application.

Actions

Suppose you have a web page with a form and one or more buttons. Normally, when the form is submitted, a method such as Servlet's respondToPost() or Page's writeHTML(), will be invoked. However, you may find it more useful to bind the button to a specific method of your servlet such as new(), remove() etc. to implement the command, and reserve writeHTML() for displaying the page and the form that invokes these methods. Note that your "command methods" can then invoke writeHTML() after performing their task.

The action feature of Page let's you do this. The process goes like this:

  1. Add buttons to your HTML form of type submit and name _action_. For example:

    <input name="_action_" type="submit" value="New">
    <input name="_action_" type="submit" value="Delete">
    
  2. Alternately, name the submit button _action_methodName. For example:

    <input name="_action_New" type="submit" value="Create New Item">
    
  3. Add an actions() method to your class to state which actions are valid. (If WebKit didn't force you to do this, someone could potentially submit data that would cause any method of your servlet to be run). For example:

    def actions(self): 
        return SuperClass.actions(self) + ['New', 'Delete']
    
  4. Now you implement your action methods.

The ListBox example shows the use of actions (in WebKit/Examples/ListBox.py)

Plug-ins

A plug-in is a software component that is loaded by WebKit in order to provide additional WebKit functionality without necessarily having to modify WebKit's source.

The most infamous plug-in is PSP (Python Server Pages) which ships with Webware.

Plug-ins often provide additional servlet factories, servlet subclasses, examples and documentation. Ultimately, it is the plug-in author's choice as to what to provide and in what manner.

Technically, plug-ins are Python packages that follow a few simple conventions in order to work with WebKit. See Creating Plugins for information about writing your own.

How do I develop an app?

The answer to that question might not seem clear after being deluged with all the details. Here's a summary:

  • Make sure you can run the WebKit AppServer. See the Install Guide for more information.
  • Read the source to the examples (in WebKit/Examples), then modify one of them to get your toes wet.
  • Create your own new example from scratch. Ninety-nine percent of the time you will be subclassing the Page class.
  • Familiarize yourself with the class docs in order to take advantage of classes like Page, HTTPRequest, HTTPResponse and Session. Unfortunately, I couldn't get generated class docs working for this release, so you'll have to resort to breezing through the source code which is coupled with documentation strings. Read the examples first.
  • With this additional knowledge, create more sophisticated pages.
  • If you need to secure your pages using a login screen, you'll want to look at the SecurePage, LoginPage, and SecureCountVisits examples in WebKit/Examples. You'll need to modify them to suit your particular needs.
  • Contribute enhancements and bug fixes back to the project. :-)
  • The Webware user community is quite supportive and friendly:

Known Bugs

Known bugs and future work in general are documented in Future.

Credit

Authors: Chuck Esterbrook, Jay Love, Geoff Talvola. @@ ib: We should add more names.

Many people, mostly on the webware-discuss mailing list, have provided feedback and testing.

The design was inspired by both Apple's WebObjects and Java's Servlets.