TAL Overview

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

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.

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

TAL Namespace

The TAL namespace URI and recommended alias are currently defined as:

      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.

Zope does not require an XML namespace declaration when creating templates with a content-type of text/html. However, it does require an XML namespace declaration for all other content-types.

TAL Statements

These are the 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:on-error - handle errors.
• tal:repeat - repeat an element.
• tal:replace - replace the content of an element and remove the element leaving the content.

Expressions used in statements may return values of any type, although most statements will only accept strings, or will convert values into a string representation. The expression language must define a value named nothing that is not a string. In particular, this value is useful for deleting elements or attributes.

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 elements, except that the content and replace statements may not appear together.

When an element has multiple statements, they are executed in this order:

1. define
2. condition
3. repeat
4. content or replace
5. attributes
6. omit-tag

Since the on-error statement is only invoked when an error occurs, it does not appear in the list.

The reasoning behind this ordering goes like this: You often want to set up variables for use in other statements, so define comes first. The very next thing to do is decide whether this element will be included at all, so condition is next; since the condition may depend on variables you just set, it comes after define. It is valuable be able to replace various parts of an element with different values on each iteration of a repeat, so repeat is next. It makes no sense to replace attributes and then throw them away, so attributes is last. The remaining statements clash, because they each replace or edit the statement element.

See Also

TALES Overview

METAL Overview

tal:attributes

tal:define

tal:condition

tal:content

tal:omit-tag

tal:on-error

tal:repeat

tal:replace