Template Attribute Language

The Template Attribute Language (TAL) is a templating language aimed to generate HTML and XML pages. Its main goal is to simplify the collaboration of programmers and designers by templates being proper HTML (or XML, respectively) which can be worked on using common design tools.

TAL was created for Zope but is used in other Python-based projects as well.

Overview

TAL Templates are often made from result pages with the variable content being replaced by special attributes; e.g., <h1 tal:content="string:Some altogether different headline"> The headline visible to the designer </h1>would be treated by a common HTML design tool as <h1> The headline visible to the designer </h1>while the application server, evaluating the tal:content attribute, would yield <h1> Some altogether different headline </h1>Of course, string literals are not the most common nor the most useful use case; the syntax of the possible attribute values is property of the TAL Expression Syntax (TALES) language. In the Python-based implementations, it is e.g. possible to use Python expressions.

Using METAL (Macro Expansion TAL) it is possible to re-use code from other templates.

When generating XML documents, it is important to specify the XML namespace xmlns:tal="http://xml.zope.org/namespaces/tal"

Attributes

The following attributes are used, normally prefixed by "tal:":

; define: creates local variables, valid in the element bearing the attribute (including contained elements); condition: decides whether or not to render the tag (and all contained text); repeat: creates a loop variable and repeats the tag iterating a sequence, e.g. for creating a selection list or a table; content: replaces the content of the tag; replace: replaces the tag (and therefore is not usable together with content or attributes); attributes: replaces the given attributes (e. g. by using tal:attributes="name name; id name" the name and id attributes of an input field could be set to the value of the variable "name"); omit-tag: allows to omit the start and end tag and only render the content if the given expression is true.; on-error: if an error occurs, this attribute works like the content tag.

If a tag has more than one TAL attributes, they are evaluated in the above (fairly logical) order.

In cases when no tag is present which lends itself to take the attributes, special TAL tags can be used, making the "tal:" prefix optional. e.g.: <tal:if condition="context/itemlist"> ... </tal:if>would cause the code inside the tal:if tags to be used whenever the context (whatever the application server defines the context to be, e.g. an object) contains variable "itemlist" with a "true" value, e.g. a list containing at least one element. The identifier following the colon is arbitrary; it simply needs to be there, and to be the same for the opening and closing tag.

Usage

TAL/TALES/METAL are used by the following projects:
* Zope (web application server)
* Roundup (issue tracker)
* Simplicity PHP framework

Other implementations

Besides the original Zope implementation, there are (not exhaustive):

Python

* SimpleTAL (http://www.owlfish.com/software/simpleTAL/, [http://www.owlfish.com/software/simpleTAL/tal-guide.html TAL-guide] )
* OpenTAL (http://savannah.nongnu.org/projects/opental/)
* ZPT (http://zpt.sourceforge.net/), a standalone version of Zope Page Templates

JavaScript

* jstal (http://code.google.com/p/jstal/)

Java

* JPT: Java Page Templates (http://christophermrossi.com/jpt/)
* JavaZPT (http://javazpt.sourceforge.net/)

Perl

* PETAL, the Perl Template Attribute Language (http://search.cpan.org/dist/Petal/)
* Template-TAL (http://search.cpan.org/dist/Template-TAL/)

PHP

* PHPtal (http://phptal.motion-twin.com)
* PHPtal (http://phptal.bukox.pl) - Polish site

XSL

* [http://wiki.flux-cms.org/display/FLX/Templates+XSLTAL XSLTal] , transforming TAL via XSLT to XSLT; http://svn.bitflux.ch/repos/public/popoon/trunk/components/transformers/xsltal/tal2xslt.xsl

External links

* [http://www.zope.org/Documentation/Books/ZopeBook/2_6Edition/AppendixC.stx Zope Page Templates Reference] (Zope Book)
* [http://wiki.zope.org/ZPT/TALSpecification14 TAL specification, v1.4]