Everything about The Template Attribute Language totally explained
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; for example,
<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's for example possible to use Python expressions.
Using
METAL (Macro Expansion TAL) it's possible to re-use code from other templates.
When generating XML documents, it's 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, for example for creating a selection list or a table
; content » replaces the content of the tag
replace » replaces the tag (and therefore isn't 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're 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. for example:
<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, for example an object) contains variable "itemlist" with a true value, for example 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:
Other implementations
Besides the original
Zope implementation, there are (not exhaustive):
Python
SimpleTAL (http://www.owlfish.com/software/simpleTAL/, TAL-guide
)
OpenTAL (http://savannah.nongnu.org/projects/opental/)
ZPT (http://zpt.sourceforge.net/), a standalone version of Zope Page Templates
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.pl/) - Polish site
XSL
XSLTal, transforming TAL via XSL to XSLT; http://svn.bitflux.ch/repos/public/popoon/trunk/components/transformers/xsltal/tal2xslt.xslFurther Information
Get more info on 'Template Attribute Language'.
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