Expressions (TALES)

The Template Attribute Language Expression Syntax (TALES) standard describes expressions that supply Template Attribute Language (TAL) and Macros (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
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.


These are the TALES expression types supported by default in SharpTAL:

  • csharp - execute a C# expression
  • string - format a string


if you do not specify a prefix within an expression context, SharpTAL assumes that the expression is a csharp expression.

Built-in Names

These are the names always available to TALES expressions in SharpTAL:

  • 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 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.

csharp expressions


Python expression syntax:

Any valid C# language expression


C# expressions are executed natively within the translated template source code.

string expressions


String expression syntax:

string_expression ::= ( plain_string | [ varsub ] )*
varsub            ::= ( '${ Expression }' )


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 ${expression}, where expression is a TALES-expression. The escaped string value of the expression is inserted into the string.


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


Basic string formatting:

<span tal:replace="string:${what} and ${that}">
  Spam and Eggs

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

Including a dollar sign:

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

Including operator ${...}:

<p tal:content="string:The expression operator: \${cost}">
  cost: $42.00