Template Attribute Language Expression Syntax (TALES)

The Template Attribute Language Expression Syntax (TALES) standard describes expressions that supply Template Attribute Language (TAL) and Macro Expansion Template Attribute Language (METAL) with data. TALES is one possible expression syntax for these languages, but they are not bound to this definition. Similarly, TALES could be used in a context having nothing to do with TAL or METAL.

TALES expressions are described below with any delimiter or quote markup from higher language layers removed. Here is the basic definition of TALES syntax:

Expression  ::= [type_prefix ':'] String
type_prefix ::= Name

Here are some simple examples:

1 + 2
None
string:Hello, ${view.user_name}

The optional type prefix determines the semantics and syntax of the expression string that follows it. A given implementation of TALES can define any number of expression types, with whatever syntax you like. It also determines which expression type is indicated by omitting the prefix.

TALES Expression Types

These are the TALES expression types supported by default in chameleon.zpt:

  • python - execute a Python expression
  • string - format a string

Note

if you do not specify a prefix within an expression context, chameleon.zpt` assumes that the expression is a python expression.

There’s a mechanism to allow fallback to alternative expressions, if one should fail (raise an exception). The pipe character (‘|’) is used to separate two expressions.

Warning

The reference implementation of ZPT has a number of expression types that are closely tied to the Zope framework.

Built-in Names

These are the names always available to TALES expressions in chameleon.zpt:

  • default - special value used to specify that existing text or attributes should not be replaced. See the documentation for individual TAL statements for details on how they interpret default.
  • repeat - the repeat variables; see tal:repeat: Repeat an element for more information.
  • template - reference to the template which was first called; this symbol is carried over when using macros.
  • macros - reference to the macros dictionary that corresponds to the current template.

python expressions

Syntax

Python expression syntax:

Any valid Python language expression

Description

Python expressions are executed natively within the translated template source code. There is no built-in security apparatus.

string expressions

Syntax

String expression syntax:

string_expression ::= ( plain_string | [ varsub ] )*
varsub            ::= ( '$' Variable ) | ( '${ Expression }' )
plain_string      ::= ( '$$' | non_dollar )*
non_dollar        ::= any character except '$'

Description

String expressions interpret the expression string as text. If no expression string is supplied the resulting string is empty. The string can contain variable substitutions of the form $name or ${expression}, where name is a variable name, and expression is a TALES-expression. The escaped string value of the expression is inserted into the string.

Note

To prevent a $ from being interpreted this way, it must be escaped as $$.

Examples

Basic string formatting:

<span tal:replace="string:$this and $that">
  Spam and Eggs
</span>

<p tal:content="string:${request.form['total']}">
  total: 12
</p>

Including a dollar sign:

<p tal:content="string:$$$cost">
  cost: $42.00
</p>