Zope
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- This article is about an application server. For the fish known as "Zope", see Abramis ballerus.
Zope is an open-source, object-oriented web application server written in the programming language Python. Zope stands for "Z Object Publishing Environment." It can be almost fully managed with a web-based user interface. Zope publishes on the web Python objects that are typically persisted in an object database, ZODB. Basic object types, such as documents, images, page templates, are available for the user to create and manage through the web. Specialized object types, such as wikis, blogs, and photo galleries, are available as third-party add-ons (called products), and there is a thriving community of small businesses creating custom web applications as Zope products.
There are two major generations of the software in use today. As of October 2006, Zope 2.10.0 is the latest stable release of Zope 2 codebase, and Zope 3.3.0 is the latest release of Zope 3. Zope is distributed under the terms of the Zope Public License, a free software license.
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[edit] History
The Zope Corporation was formed in 1995 in Fredericksburg, VA, as a joint venture with InfiNet (a Gannett/ Knight Ridder/ Landmark Communications joint newspaper chain venture.) The Company developed a classified advertisement engine for the Internet, then in 1997 became an independently owned private company. At the behest of its largest investor, Opticality Ventures, Zope Corporation released its software as free software, concentrating on providing premier customization services. The company has a world-class team of software engineers led by CTO Jim Fulton. PythonLabs, creators of Python, (foundation of Zope), became part of the company in the year 2000. Python founder Guido Van Rossum is no longer employed by Zope, however.
What is now known as Zope 2 started off with the merger of three separate software products – Bobo, Document Template, and BoboPOS – into the Principia application server. The authoring company, Digital Creations, open-sourced their flagship product in 1998, influenced by their venture capitalist, Hadar Pedhazur. This product was renamed "Zope" with this release.
In November 2004, Zope 3 was released. Zope 3 is a complete rewrite that only preserves the original ZODB object database. The design of Zope 3 is driven by the needs of large companies, the clients of Zope Corporation, for complex business schemas. It is directly intended for enterprise web application development using the newest development paradigms. Zope 3 is, however, not compatible with Zope 2, so you cannot run Zope 2 applications on Zope 3. Therefore, an effort (called Five) to merge the new Zope 3 paradigms into Zope 2 has been ongoing since Zope 2.8. Its aim is to allow developers to write applications that run under both Zope 3 and Zope 2.
But the history of Zope is much richer than the high-level perspective allows. Literally hundreds of open-source Zope products have been released on the web, and many have been absorbed into the core Zope 2 product. Others have withered away in unmaintained obscurity.
[edit] Technical features
A Zope website is composed of objects in an object database as opposed to files, as is usual with many other web server systems. This approach allows users to harness the advantages of object technologies, such as encapsulation. Zope maps URLs to objects using the containment hierarchy of such objects; methods are considered to be contained in their objects as well.
Zope comes with the Zope Object Database, which transparently persists (Python) objects in a transactional database.
One particularly innovative feature of Zope is its widespread use of acquisition. Acquisition is a technique parallel to class inheritance, in which objects "inherit" behavior from their context in a composition hierarchy, as opposed to their class in a class hierarchy. This allows certain ways to structure source code that are otherwise harder to accomplish, and can encourage application decomposition. A common use is in structuring the way layout elements are used in a web page. Acquisition as implemented in Zope 2 is also perceived as a source of bugs, as it enables many unexpected behaviours. The use of acquisition has been severely narrowed in Zope 3.
Zope provides several mechanisms for HTML templating: Dynamic Template Markup Language (DTML), and Zope Page Templates (ZPT). DTML is a tag-based language which allows to implement simple scripting in the templates. DTML has provisions for variable inclusion, conditions, and loops. However, DTML has major drawbacks: DTML tags interspersed with HTML form non-valid HTML documents, and careless inclusion of logic into templates results in very unreadable code. ZPT is a technology that fixes these shortcomings. ZPT templates can be either well-formed XML documents or HTML documents, in which all special markup is presented as attributes in the TAL (Template Attribute Language) namespace. ZPT offers just a very limited set of tools for conditional inclusion and repetition of XML elements, thus the templates are usually quite simple, with most logic implemented in Python code. One significant advantage of ZPT templates is that they can be edited in most graphical HTML editors. ZPT also offers direct support for internationalization.
Zope 2 is the base behind the Nuxeo CPS, Plone, and Silva content management systems, as well as the base behind ERP5 open source enterprise resource planning. However, in late 2006 Nuxeo announced that future versions of its CMS would be implemented in Java.
[edit] Zope 3
A new version of Zope, Zope 3, has been in development for some years. Zope 2 has proven itself as a useful framework for web applications development, but its use revealed some shortcomings. To name a few, creating Zope 2 products involves copying a lot of boilerplate code – "magic" code – that just has to be there, and the domain logic is highly coupled with the presentation logic, so creating and maintaining tailored versions is difficult. Zope 3 is a rewrite of the software that attempts to address these shortcomings while retaining the advantages of Zope that led to its popularity. Zope 3 is based on a component architecture that makes it easy to mix software components of various origins (Python, Java, others...). The first production release of the new software, Zope X3 3.0.0, was released on November 6, 2004. The current production release, Zope 3.3.0, was released on September 27, 2006.
[edit] Zope Page Templates
As mentioned previously, Zope Page Templates are themselves XHTML documents which means they can be viewed and edited using XHTML compliant tools (which is a big benefit compared to other template languages used for web applications). However, these page templates are not meant to be rendered as is. Instead they are marked up with additional elements and attributes in special XML namespaces (see below). This additional information is used to describe how the page template should ultimately be processed.
Here are some basic examples. To conditionally include a particular element, like a div element, simply add the tal:condition attribute to the element as follows:
<div tal:condition="..."> ... </div>
To control what appears inside an element, use the tal:content attribute like this:
<h1><span tal:content="..."/></h1> ...
Finally, to introduce or replace values of attributes use the tal:attributes attribute as follows: The power of python could also be utilised to dynamically alter the href at runtime.
<a href="" tal:attributes="python: 'href http://someurl.com'">...</a>
Because Zope Page Templates are themselves XHTML not only can they be edited using normal HTML editors but they can also be checked for XHTML compliance in template form. As a result, you can be fairly confident that these templates will be expanded into proper XHTML automatically.
This a very cursory explanation of Zope Page Templates. The behavior of Zope Page Templates is almost completely described by a template language, fixed on TAL, TALES, and METAL specifications:
- Tag Attribute Language (TAL),
- Template Attribute Language Expression Syntax (TALES),
- Macro Expansion Template Attribute Language (METAL).
