Zope

This article is about an application server. For the fish known as "Zope", see Abramis ballerus.

Zope 2

Screenshot  File:Zope-screenshot.png Zope management interface in a web browser window.
Developer(s)	Zope Corporation
Initial release	1998 (1998)
Stable release	2.12.2 / December 22, 2009
Written in	Python
Operating system	Cross-platform
Development status	Active
Type	web application server
License	Zope Public License
Website	http://zope2.zope.org/

Zope 3

Screenshot
Developer(s)	Zope Corporation
Initial release	2004 (2004)
Stable release	3.4 / January 30, 2009
Written in	Python
Operating system	Cross-platform
Development status	Active
Type	web application server
License	Zope Public License
Website	http://www.zope.org/

Zope is a free and open-source, object-oriented web application server written in the Python programming language. 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, and 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.^{[citation needed]}

The Zope community has the last years spawned several different related web frameworks with slightly different aims and principles but often sharing a lot of the philosophy, people and code. Zope 2 is still the most widespread of these frameworks, largely thanks to the content management system Plone, which runs on Zope 2. Zope 3 is less widespread but underlies several large sites, like for example Launchpad. Grok was started as a more programmer friendly framwork, "Zope 3 for cavemen", and during 2009 BFG gained popularity in the Zope community as a minimalistic framework based on Zope principles.

History

The Zope Corporation was formed in 1995 in Fredericksburg, VA under the name Digital Creations, as a joint venture with InfiNet (a 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's software engineers are led by CTO Jim Fulton. PythonLabs, creators of Python, became part of the company in the year 2000 (Python founder Guido van Rossum left Zope Corp in 2003).

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, re-released their flagship product as free and open-source software 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 preserves only 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. It was originally intended to introduce a backwards compatibility layer so that Zope 2 software would run on Zope 3. Instead a module known as Five introduced the new Zope 3 paradigms into Zope 2, although full compatibility isn't possible that way either.

The existance of two incompatible web frameworks called Zope has caused a lot of confusion. In response Zope 3 was in January 2010 renamed "BlueBream".

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. In Zope 3 the use of acquisition has been made explicit. You only acquire data if you explicitly say that it is the acquired data you want.

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 implementation of 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 Plone, and Silva content management systems, as well as the base behind ERP5 open source enterprise resource planning.

Zope 3 / BlueBream

Main article: Zope 3

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 built-in management interface is difficult to modify or replace. 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 written in Python. Although originally intended as a replacement for Zope 2, the Zope 3 component architecture was instead been backported to Zope 2 starting with Zope 2.8. Also many Zope platforms such as Plone are going through the same type of piece by piece rewriting.

The first production release of the new software, Zope X3 3.0.0, was released on November 6, 2004. The current production release, Zope 3.4.0, was released on January 30, 2009. The main effort for Zope 3.4.0 was so called "Eggification", the splitting up of all the separate pieces of Zope 3 in to separate packages, providing multiple independent infrastructure components that could be used by other projects. This means new versions of Zope 3 are released almost continuously. Developers tend to use the last version of all packages and simply fix the versions on release. Official releases has therefore become less useful, as a release is now only a list of packages and their versions that has been tested together and known to be stable. Such a list is called a Known Good Set, KGS.

Zope Toolkit

As a result of the eggification of Zope 3, there is now many independent Python packages used and developed as a part of Zope 3, and although many of these are useable outside of Zope 3, many are not. To make it clearer which ones are independently usable, and also improve the re-usability of the packages the Zope Toolkit (ZTK) project was started. Thus the Zope Toolkit works as the base where each of the Zope-world frameworks builds on. Zope 2.12 is the first release of a web framework that builds on Zope Toolkit, and Grok and BlueBream are set to have releases based on the ZTK during 2010.

Grok

In 2006 the Grok project was started by a number of Zope 3 developers who wanted to make Zope 3 technology more agile in use and more accessible to newcomers. Grok has since then seen regular releases and its core technology (Martian, grokcore.component) is also finding uptake in other Zope 3 and Zope 2 based projects.

Zope Page Templates

As mentioned previously, Zope Page Templates are themselves XHTML documents which means they can be viewed and edited using normal HTML editors or XHTML compliant tools (a big advantage compared to other template languages used for web applications). Templates can also be checked for XHTML compliance so you can be fairly confident that they will automatically expand into proper XHTML.

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="href python:'http://someurl.com/%s'%someobject">...</a>

This is 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:

• Template Attribute Language (TAL),
• Template Attribute Language Expression Syntax (TALES),
• Macro Expansion Template Attribute Language (METAL).

Zope Management Interface

The Zope application server features a web-based management interface that enables an administrative user to change many aspects about the way Zope behaves. It was this ease of customizability that gave Zope both its popularity and its infamous shortcomings described above.

See also

• Content management
• Django
• ERP5
• Plone
• Web content management
• Zope Content Management Framework
• Zwiki

External links

• Zope website
• Zope 2 wiki
• Zope 3 wiki
• Zope Corporation
• The Zope Book
• template (ZPT) Examples
• more Zope wikis