Last month we took an initial look at the open-source Zope application server. In particular, we saw how you can use Zope's DTML (dynamic template markup language) tags to create simple dynamic sites, as well as how you can manage a web site using nothing more than a web browser.
But anyone who has worked with DTML knows that it ceases to be wonderful when you want to create something relatively complex. DTML is best when it is used sparingly or when its functionality is obvious; writing pages of DTML that contain a half-dozen nested conditional (<dtml-if>) tags quickly becomes unreadable and difficult to maintain, not to mention very nonmodular.
Another problem is that DTML exists inside of individual documents, rather than in a central location. If we want to reuse functionality in multiple places, then we must copy our DTML methods and documents. This means that when we want to add or change some functionality, we must go through each of the copies and modify them as well.
The solution to this problem is the Zope product. Each Zope product is actually an object class (or a set of classes) that can be instantiated any number of times in our web site.
This month, we look at Zope products, which form the core of Zope's flexibility. After installing and working with some existing products, we will write our own simple product in Python.
A Zope product is a package of code, graphics and DTML that provides a piece of reusable functionality. For example, if we were interested in creating a simple page that displays the current time, we could create a DTML document:
<p>It is now <dtml-var name="ZopeTime" fmt="fCommon">.</p>
But what if we want to expand our page, displaying a weather forecast retrieved via HTTP from another server? DTML is not the answer here; even if we could use it to create our custom functionality, the result would be difficult to manage, as well as ugly to write. Because Zope products are written in Python, they can use any Python module they like, displaying their output in HTML or any other compatible format.
Because each product is treated as a single entity, we can install and remove them as a single unit even if it defines and uses a number of classes. However, this doesn't mean that each product stands on its own; on the contrary, it is possible for one product to use functionality provided by another product.
In addition to products and DTML documents, Zope provides two other means for creating dynamic content: Python scripts (implemented by a product, no less) allow us to write and use small Python programs within Zope. We also can create, edit and use new products using a system known as ZClasses. ZClasses allow you to create new products (and their associated classes) using nothing more than your web browser and DTML.
While these four options provide a great deal of flexibility, deciding which one to use can sometimes be difficult for beginning Perl programmers. Beehive's The Book of Zope, which I review in this issue of Linux Journal, suggests using ZClasses at the beginning of a project, migrating the code to a full-fledged product after everyone has agreed upon a design. The more complex your functionality is, the more likely it is that you will want to use or write a product rather than rely on DTML and Python scripts.
You can run almost every aspect of Zope products via the Zope management screen, which you can reach via the /manage URL of your Zope server. Click on the control panel link in the left-hand frame to bring up the Zope control panel and on the Product Management link in the main frame to bring up the product management screen.
You should see a list of Zope products, along with a button marked “Add product” at the top of the screen. Products that you can modify through the Web (including ZClasses, which we briefly mentioned above) are identified with an open box, whereas standard Zope products have a closed-box icon. A closed box simply means that you cannot modify the product itself via the Web. However, most products will let you customize them by setting one or more properties via a web-based interface. But the product itself remains unchanged, unless you modify the source code.
Each product is actually a directory under your Zope installation directory in lib/python/Products. The Sessions product is under lib/python/Products/Sessions, while the Transience product is in lib/python/Products/Transience. (I installed Zope under /usr/local/zope/ on my system, so Sessions is actually in /usr/local/zope/lib/python/Products/Sessions/.) A product directory contains Python code, text files and directories, including:
__init__.py: this is what Zope scans and executes when it loads your module. Among other things, the initialize method in __init__.py invokes context.registerClass, which (as its name implies) tells Zope that your product exists, what text to display in the Add menu on the /manage screen (with the meta_type parameter) and how to create a new instance of your product when the Add button has been pressed (with the constructors parameter).
README.txt: as its name implies, this is the README file for a particular product. Clicking on a product name from within the control panel will display a README tab, among others. This tab allows you to look at README.txt without having to look at the filesystem. If the product directory contains no file named README.txt, then no README tab will appear at the top of the screen.
version.txt: this file contains the name and current version number of your product, separated by minus signs (-). Version 1.2.3 of the product Foo thus will have a version.txt with the following contents: Foo-1-2-3. This version information is displayed in the control panel.
Help files: a product may contain a help directory, which contains the text displayed by Zope when you click on the help link. Help files are often written using structured text, a minimalist formatting system similar in spirit to Perl's POD documentation system. Structured text is easy to write with a simple text editor and equally easy to read with a standard Linux tool like less.
Zope only looks at the current list of products when it starts up. This means that if you install a new product, you will need to restart your Zope server. This is done most easily from within the control panel.
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