Template Attribute Language (TAL) Reference

The Template Attribute Language (TAL) is an attribute language used to create dynamic XML-like content. It allows elements of a document to be replaced, repeated, or omitted.

An attribute language is a programming language designed to render documents written in XML markup. The input XML must be well-formed. The output from the template is usually XML-like but isn't required to be well-formed.

The statements of the language are document tags with special attributes, and look like this:
<p namespace:command="argument">Some Text</p>

In the above example, the attribute namespace:command="argument" is the statement, and the entire paragraph tag is the statement's element. The statement's element is the portion of the document on which this statement operates.

Each statement has three parts: the namespace prefix, the name, and the argument. The prefix identifies the language, and must be introduced by an XML namespace declaration in XML and XHTML documents, like this:
<root xmlns:namespace="http://example.com/namespace">

The statements of TAL are XML attributes from the TAL namespace. These attributes can be applied to an XML or HTML document in order to make it act as a template.

The TAL namespace URI is currently defined as:
<root xmlns:tal="http://xml.zope.org/namespaces/tal">

This is not a URL, but merely a unique identifier. Do not expect a browser to resolve it successfully. This definition is required in every file that uses ZPT. For example:
<div xmlns="http://www.w3.org/1999/xhtml"
   .... rest of the template here ...

TAL Statements

A TAL statement has a name (the attribute name) and an argument (the attribute value). For example, a tal:content statement might look like tal:content="string:Hello". The element on which a statement is defined is its statement element. Most TAL statements are expressions, but the syntax and semantics of these expressions are not part of TAL.

NOTE: TALES is used as the expression language for the "stuff in the quotes" typically. TALES is documented separately.

These are the available TAL statements:
  • tal:attributes - dynamically change element attributes.
  • tal:define - define variables.
  • tal:condition - test conditions.
  • tal:content - replace the content of an element.
  • tal:omit-tag - remove an element, leaving the content of the element.
  • tal:repeat - repeat an element.
  • tal:replace - replace the content of an element and remove the element leaving the content.

Order of Operations

When there is only one TAL statement per element, the order in which they are executed is simple. Starting with the root element, each element's statements are executed, then each of its child elements is visited, in order, to do the same.

Any combination of statements may appear on the same element, except that the tal:content and tal:replace statements may not be used on the same element.

TAL does not use use the order in which statements are written in the tag to determine the order in which they are executed. When an element has multiple statements, they are executed in this order:
  1. tal:define
  2. tal:condition
  3. tal:repeat
  4. tal:content or tal:replace
  5. tal:omit-tag
  6. tal:attributes

There is a reasoning behind this ordering. Because users often want to set up variables for use in other statements contained within this element or subelements, tal:define is executed first. tal:condition follows, then tal:repeat , then tal:content or tal:replace. Finally, before tal:attributes, we have tal:omit-tag (which is implied with tal:replace).

tal:attributes - Replace element attributes


    argument             ::= attribute_statement [';' attribute_statement]*
    attribute_statement  ::= attribute_name expression
    attribute_name       ::= [namespace-prefix ':'] Name
    namespace-prefix     ::= Name


The tal:attributes statement replaces the value of an attribute (or creates an attribute) with a dynamic value. The value of each expression is converted to a string, if necessary.

NOTE: You can qualify an attribute name with a namespace prefix, for example html:table, if you are generating an XML document with multiple namespaces.

If an attribute expression evaluates to null, then that attribute is deleted from the statement element.
If the expression evaluates to the symbol default (a symbol which is always available when evaluating attributes), its value is defined as the default static attribute value.

If you use tal:attributes on an element with an active tal:replace command, the tal:attributes statement is ignored.

If you use tal:attributes on an element with a tal:repeat statement, the replacement is made on each repetition of the element, and the replacement expression is evaluated fresh for each repetition.


Replacing a link:
<a href="/sample/link.html"
    tal:attributes="href context.url()">

Replacing two attributes:
<textarea rows="80" cols="20"
    tal:attributes="rows request.rows();cols request.cols()">

tal:condition - Conditionally insert or remove an element


    argument ::= expression


The tal:condition statement includes the statement element in the template only if the condition is met, and omits it otherwise.
If its expression evaluates to a true value, then normal processing of the element continues, otherwise the statement element is immediately removed from the template.
For these purposes, the value nothing is false, and default has the same effect as returning a true value.

NOTE: SharpTAL considers null, zero, empty strings, empty sequences, empty dictionaries false; all other values are true, including default.


Test a variable before inserting it:
<p tal:condition="request.message"
   message goes here

Testing for odd/even in a repeat-loop:
<div tal:repeat="item Enumerable.Range(0, 10)">
  <p tal:condition="repeat.item.even">Even</p>
  <p tal:condition="repeat.item.odd">Odd</p>

tal:content - Replace the content of an element


    argument ::= (['text'] | 'structure') expression


Rather than replacing an entire element, you can insert text or structure in place of its children with the tal:content statement.
The statement argument is exactly like that of tal:replace, and is interpreted in the same fashion.
If the expression evaluates to null, the statement element is left childless. If the expression evaluates to default, then the element's contents are unchanged.

The default replacement behavior is text, which replaces angle-brackets and ampersands with their HTML entity equivalents.
The structure keyword passes the replacement text through unchanged, allowing HTML/XML markup to be inserted.
This can break your page if the text contains unanticipated markup (eg. text submitted via a web form), which is the reason that it is not the default.


Inserting the user name:
<p tal:content="user.getUserName()">Fred Farkas</p>

Inserting HTML/XML:
<p tal:content="structure context.getStory()">marked <b>up</b>content goes here.</p>

tal:define - Define variables


    argument             ::= attribute_statement [';' attribute_statement]*
    attribute_statement  ::= [context] variable_name expression
    context              ::= global | local | set
    variable_name        ::= Name


The tal:define statement defines variables.

When you define a local variable in a statement element, you can use that variable in that element and the elements it contains.

You can define a local variable in a special statement without local scope <tal:omit-scope />. Variables defined in <tal:omit-scope />, can be used in parent element and the elements it contains.

If the expression associated with a variable evaluates to null, then that variable has the value null, and may be used as such in further expressions.
Likewise, if the expression evaluates to default, then the variable has the value default, and may be used as such in further expressions.


Defining a global variable:
<tal:tag tal:define='global company_name '"My Company"'>

Defining a local variable:
<tal:tag tal:define='company_name "My Company"'>

Defining two local variables, where the second depends on the first:
<tal:tag tal:define="mytitle context.title; tlen mytitle.Length">

Set allready defined local variable:
<tal:tag tal:define="set mytitle context.title">

Use functions that return void:
<tal:tag tal:define='tmp 0;;Console.WriteLine("One!");;Console.WriteLine("Two!");;'>

Defining and using a local variables in special element without local scope:
  <tal:omit-scope tal:define='Name "Sandra"' />
  <tal:omit-scope tal:define='Age 23' />
  <i tal:content="string:Hello, my name is ${Name}. I'm ${Age} years old."></i>

tal:omit-tag - Remove an element, leaving its contents


    argument ::= [ expression ]


The tal:omit-tag statement leaves the contents of an element in place while omitting the surrounding start and end tags.

If the expression evaluates to a false value, then normal processing of the element continues and the tags are not omitted. If the expression evaluates to a true value, or no expression is provided, the statement element is replaced with its contents.

NOTE: null, zero, empty strings, empty sequences, empty dictionaries are false; all other values are true, including default.


Unconditionally omitting a tag:
<div tal:omit-tag="" comment="This tag will be removed">
  <i>...but this text will remain.</i>

Conditionally omitting a tag:
<b tal:omit-tag="bold == false">I may be bold.</b>
The above example will omit the b tag if the variable bold is false.

Creating ten paragraph tags, with no enclosing tag:
<span tal:repeat="n Enumerable.Range(0, 10)"
  <p tal:content="n">1</p>

tal:repeat - Repeat an element


    argument      ::= variable_name expression
    variable_name ::= Name


The tal:repeat statement replicates a sub-tree of your document once for each item in a sequence. The expression should evaluate to a sequence.
If the sequence is empty, then the statement element is deleted, otherwise it is repeated for each value in the sequence.
If the expression is default, then the element is left unchanged, and no new variables are defined.

The variable_name is used to define a local variable and a repeat variable. For each repetition, the local variable is set to the current sequence element, and the repeat variable is set to an iteration object.

Repeat Variables

You use repeat variables to access information about the current repetition (such as the repeat index). The repeat variable has the same name as the local variable, but is only accessible through the built-in variable named repeat.

The following information is available from the repeat variable:
  • index - repetition number, starting from zero.
  • number - repetition number, starting from one.
  • even - true for even-indexed repetitions (0, 2, 4, ...).
  • odd - true for odd-indexed repetitions (1, 3, 5, ...).
  • start - true for the starting repetition (index 0).
  • end - true for the ending, or final, repetition.
  • length - length of the sequence, which will be the total number of repetitions.
  • letter - repetition number as a lower-case letter: "a" - "z", "aa" - "az", "ba" - "bz", ..., "za" - "zz", "aaa" - "aaz", and so forth.
  • Letter - upper-case version of letter.
  • roman - repetition number as a lower-case roman numeral: "i", "ii", "iii", "iv", "v", etc.
  • Roman - upper-case version of roman.

You can access the contents of the repeat variable using dictionary, e.g. repeat"item".start.


Iterating over a sequence of strings:
<p tal:repeat='txt new List<string>() { "one", "two", "three" }'>
   <span tal:replace="txt" />

Inserting a sequence of table rows, and using the repeat variable to number the rows:
  <tr tal:repeat="item here.cart">
      <td tal:content='repeat["item"].number'>1</td>
      <td tal:content="item.description">Widget</td>
      <td tal:content="item.price">$1.50</td>

Nested repeats:
<table border="1">
  <tr tal:repeat="row Enumerable.Range(0, 10)">
    <td tal:repeat="column Enumerable.Range(0, 10)">
      <span tal:define='x repeat["row"].number; 
                        y repeat["column"].number; 
                        z x * y'
            tal:replace="string:${x} * ${y} = ${z}">1 * 1 = 1</span>

Insert objects. Separates groups of objects by type by drawing a rule between them:
<div tal:repeat="object objects">
  <h2 tal:condition='repeat["object"].first.meta_type'
    tal:content="object.type">Meta Type</h2>
  <p tal:content="object.id">Object ID</p>

NOTE: the objects in the above example should already be sorted by type.

tal:replace - Replace an element


    argument ::= ['structure'] expression


The tal:replace statement replaces an element with dynamic content. It replaces the statement element with either text or a structure (unescaped markup). The body of the statement is an expression with an optional type prefix.
The value of the expression is converted into an escaped string unless you provide the 'structure' prefix. Escaping consists of converting &amp; to &amp;amp;, &lt; to &amp;lt;, and &gt; to &amp;gt;.

If the expression evaluates to null, the element is simply removed. If the value is default, then the element is left unchanged.


Inserting a title::
<span tal:replace="context.title">Title</span>

Inserting HTML/XML::
<div tal:replace="structure table" />

Last edited Oct 26, 2010 at 10:08 PM by rlacko, version 4


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