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Zope is a family of free and open-source web application servers written in Python, and their associated online community. Zope stands for "Z Object Publishing Environment", and was the first system using the now common object publishing methodology for the Web. Zope has been called a Python killer app, an application that helped put Python in the spotlight.
Over the last few years, the Zope community has spawned several additional with disparate aims and principles, but sharing philosophy, people, and source code. Zope 2 is still the most widespread of these frameworks, largely thanks to the Plone content management system, which runs on Zope 2. BlueBream (earlier called Zope 3) is less widespread but underlies several large sites, including Launchpad. Grok was started as a more programmer-friendly framework, "Zope 3 for cavemen", and in 2009 Pyramid gained popularity in the Zope community as a minimalistic framework based on Zope principles.
What is now known as Zope 2 began with the merging of three separate software products – Bobo, Document Template, and BoboPOS – into the Principia application server. At the behest of its largest investor, Opticality Ventures, Principia was re-released as free software under the name Zope in 1998. Bobo, and therefore Zope, was the first Web object publishing solution.
In November 2004, Zope 3 was released. Zope 3 is a complete rewrite that preserves only the original ZODB object database. It is directly intended for enterprise Web application development using the newest development paradigms. Zope 3 is, however, not compatible with Zope 2, so Zope 2 applications do not run 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 existence of two incompatible Web frameworks called Zope has caused a lot of confusion. In response, in January 2010, Zope 3 was renamed "BlueBream". "Zope" and "blue bream" are names of a kind of fish, Ballerus ballerus.
Zope provides two mechanisms for HTML templating: Document Template Markup Language (DTML) and Zope Page Templates (ZPT). DTML is a tag-based language that allows implementation of simple scripting in the templates. DTML has provisions for variable inclusion, conditions, and loops. However, DTML can be problematic: DTML tags interspersed with HTML form non-valid HTML documents, and its use requires care when including logic into templates, to retain code readability. The use of DTML is discouraged by many leading Zope developers. ZPT is a technology that addresses the shortcomings of DTML. 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 a very limited set of tools for conditional inclusion and repetition of XML elements. Consequently, 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 underlies the Plone content management system, as well as the ERP5 open source enterprise resource planning system.
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 was a rewrite of the software that attempts to address these shortcomings while retaining the advantages of Zope that led to its popularity. BlueBream 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 Component Architecture has instead been backported to Zope 2, starting with Zope 2.8. 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 libraries underlying BlueBream have been evolving into a collection of useful libraries for web application development rather than a single, monolithic application server. BlueBream includes separate packages for interfaces, component architecture, HTTP server, publisher, Zope Object Database (ZODB), Zope Page Templates, I18N, security policy, and so on. The component architecture is used to glue these together. The component architecture is configured using a ZCML (Zope Configuration Markup Language), an XML based configuration file language.
The Zope 3 project pioneered the practice of code sprint for open source software development. Sprints are intensive development sessions when programmers, often from different countries, gather in one room and work together for a couple of days or even several weeks. During the sprints various practices drawn from agile software development are used, such as pair programming and test-driven development. Besides the goal of developing software, sprints are also useful for geographically separated developers to meet in person and attracting new people to the project. They also serve as a way for the participants to learn from each other.
BlueBream is considered a stable framework, used on production projects worldwide, most notably Launchpad.
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:
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To control what appears inside an element, use the tal:content attribute like this:
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Finally, to introduce or replace values of attributes use the tal:attributes attribute as below. You can use Python to alter the href at runtime.
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:
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