TALES Overview

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

    a/b/c
    path:a/b/c
    nothing
    path:nothing
    python: 1 + 2
    string:Hello, ${user/getUserName}

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.

If you do not specify a prefix, Zope assumes that the expression is a path expression.

TALES Expression Types

These are the TALES expression types supported by Zope:

• path expressions - locate a value by its path.
• exists expressions - test whether a path is valid.
• nocall expressions - locate an object by its path.
• not expressions - negate an expression
• string expressions - format a string
• python expressions - execute a Python expression

Built-in Names

These are the names that always available to TALES expressions in Zope:

• nothing - special value used by to represent a non-value (e.g. void, None, Nil, NULL).
• default - special value used to specify that existing text should not be replaced. See the documentation for individual TAL statements for details on how they interpret default.
• options - the keyword arguments passed to the template. These are generally available when a template is called from Methods and Scripts, rather than from the web.
• repeat - the repeat variables; see the tal:repeat documentation.
• attrs - a dictionary containing the initial values of the attributes of the current statement tag.
• CONTEXTS - the list of standard names (this list). This can be used to access a built-in variable that has been hidden by a local or global variable with the same name.
• root - the system's top-most object: the Zope root folder.
• here - the object to which the template is being applied.
• container - The folder in which the template is located.
• template - the template itself.
• request - the publishing request object.
• user - the authenticated user object.
• modules - a collection through which Python modules and packages can be accessed. Only modules which are approved by the Zope security policy can be accessed.

Note the names root, here, container, template, request, user, and modules are optional names supported by Zope, but are not required by the TALES standard.

See Also

TAL Overview

METAL Overview

exists expressions

nocall expressions

not expressions

string expressions

path expressions

python expressions