Oct. 18, 2008, 5:52 p.m.
posted by bruce
Grail: A Python-Based Web Browser
As mentioned earlier, Grail was something of a proof of concept for using Python to code large-scale Internet applications. It implements all the usual Internet protocols and works much like common browsers such as Netscape and Internet Explorer. Grail pages are implemented with the Tkinter text widgets that we met in Part III of this book.
More interestingly, the Grail browser allows applets to be written in Python. Grail applets are simply bits of Python code that live on a server but are run on a client. If an HTML document references a Python class and file that live on a server machine, Grail automatically downloads the Python code over a socket and runs it on the client machine, passing it information about the browser's user interface. The downloaded Python code may use the passed-in browser context information to customize the user interface, add new kinds of widgets to it, and perform arbitrary client-side processing on the local machine. Roughly speaking, Python applets in Grail serve the same purposes as Java applets in common Internet browsers: they perform client-side tasks that are too complex or slow to implement with other technologies such as server-side CGI scripts and generated HTML.
A Simple Grail Applet Example
Writing Grail applets is remarkably straightforward. In fact, applets are really just Python/Tkinter programs; with a few exceptions, they don't need to "know" about Grail at all. Let's look at a short example; the code in Figure simply adds a button to the browser, which changes its appearance each time it's pressed (its bitmap is reconfigured in the button callback handler).
There are two components to this page definition: an HTML file and the Python applet code it references. As usual, the grail.html HTML file that follows describes how to format the web page when the HTML's URL address is selected in a browser. But here, the APP tag also specifies a Python applet (class) to be run by the browser. By default, the Python module is assumed to have the same name as the class and must be stored in the same location (URL directory) as the HTML file that references it. Additional APP tag options can override the applet's default location.
The applet file referenced by the HTML is a Python script that adds widgets to the Tkinter-based Grail browser. Applets are simply classes in Python modules. When the APP tag is encountered in the HTML, the Grail browser downloads the Question.py source code module (Figure) and makes an instance of its Question class, passing in the browser widget as the master (parent). The master is the hook that lets applets attach new widgets to the browser itself; applets extend the GUI constructed by the HTML in this way.
Notice that nothing in this class is Grail or Internet specific; in fact, it can be run (and tested) standalone as a Python/Tkinter programtry it. When run by Grail (with the browser/page object as the master), the button appears as part of the web page instead. Either way, its bitmap changes on each press.
In effect, Grail applets are simply Python modules that are linked into HTML pages by using the APP tag. The Grail browser downloads the source code identified by an APP tag and runs it locally on the client during the process of creating the new page. New widgets added to the page (like the button here) may run Python callbacks on the client later, when activated by the user.
Applets interact with users by creating one or more arbitrary Tkinter widgets. Of course, the previous example is artificial, but notice that the button's callback handler could do anything we can program in Python: updating persistent information, popping up new user interaction dialogs, calling C extensions, using Python's networking tools, and so on.
Having said all that, I should add that Grail is no longer formally maintained, and it is now used primarily for research purposes (Guido never intended for Grail to put Netscape or Microsoft out of business). You can still find the Grail for free with a web search, and you can use it for surfing the Web or experimenting with alternative web browser concepts, but it is not the active project it was a few years ago.
If you want to code web browser applets in Python, the more common approach today is to use the Jython system described previously. Embedding Python code in HTML with the Active Scripting extension described later in this chapter is yet another way to integrate client-side code.