TALES Specification Version 1.3

This specification supercedes TALES Specification Version 1.2.

The Template Attribute Language Expression Syntax describes expressions that may be used to supply Template Attribute Language and Macro Expansion for TAL (or 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 Template Attribute Language or Macro Expansion for TAL (or METAL).

TALES expressions are described below with any delimiter or quote markup from higher language layers removed, since this is how expression strings must be passed to the TALES engine. Here is the basic definition of TALES syntax:

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

See :doc:`EBNF` for rules and terminals.

Here are some simple examples:

python: 1 + 2
string:Hello, ${username}

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.

Several expression types are required:

Required Type Prefixes

  • not - evaluate the expression string (recursively) as a full expression, and returns the boolean negation of its value. If the expression supplied does not evaluate to a boolean value, not will issue a warning and coerce the expression’s value into a boolean type based on the following rules:

    1. integer 0 is false
    2. integer > 0 is true
    3. an empty string or other sequence is false
    4. a non-empty string or other sequence is true
    5. a non-value (e.g. void, None, Nil, NULL, etc) is false
    6. all other values are implementation-dependent.

    If no expression string is supplied, an error should be generated.

  • path - interpret the expression string as the path to some object. The syntax and semantics of paths warrant their own section and thus are described below (see [Path Expressions]) If no expression string is supplied, the result is interpreted as the value nothing.

  • string - 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 ‘$name’ or ‘${name}’, where ‘name’ is an expression of type ‘path’. The escaped string value of the path expression is inserted into the string. To prevent a ‘$’ from being interpreted this way, it must be escaped as ‘$$’. Note that leading and trailing spaces in the expression are included in the value.


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

Optional Type Prefixes

  • python - interpret the expression string as restricted Python code. This code must be a legitimate python expression.

Path Expressions

A path expression consists of one or more paths separated by vertical bars (|). A path consists of one or more non-empty strings separated by slashes. The first string must be a variable name, and the remaining strings, the path segments, may contain letters, digits, spaces, and the punctuation characters underscore, dash, period, comma, and tilde.

Here is the syntax:

PathExpr  ::= Path [ '|' Path ]*
Path      ::= variable [ '/' URL_Segment ]*
variable  ::= Name

For example:

here/some-file 2001_02.html.tar.gz/foo
root/to/branch | default

When a TALES path expression is evaluated, it attempts to traverse each path, from left to right, until it succeeds or runs out of paths. To traverse a path, it first fetches the object stored in the variable. For each path segment, it traverses from the current object to the subobject named by the path segment.

Once a path has been successfully traversed, the resulting object is the value of the expression. If it is a callable object, such as a method or class, it is called. The semantics of traversal (and what it means to be callable) are implementation-dependent.

If a traversal step fails, evaluation immediately proceeds to the next path. If there are no further paths, an error results.

Since every path must start with a variable name, you need a set of starting variables that you can use to find other objects and values. PageTemplates define the variable names listed below. Since variable names are looked up first in locals, then in globals, then in this list, these names act just like builtins in Python; They are always available, but they can be shadowed by a global or local variable declaration. You can always access the builtin names explicitely by prefixing them with CONTEXTS. (e.g. CONTEXTS/root, CONTEXTS/nothing, etc).

Builtin Names in Page Templates

  • nothing - special singleton value used by TAL to represent a non-value (e.g. void, None, Nil, NULL).
  • default - special singleton value used by TAL to specify that existing text should not be replaced.
  • options - the keyword arguments passed to the template.
  • repeat - the ‘repeat’ variables (see RepeatVariable).
  • 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 builtin variable that has been hidden by a local or global variable with the same name.

Optional Names in Page Templates (used in Zope Page Templates)

  • root - the system’s top-most object.
  • here - the object to which the template is being applied.
  • container - the template’s container object.
  • template - the template itself.
  • request - the publishing request object.
  • user - the authenticated user object.
  • modules - a collection through which all Python modules and packages can be accessed. Some or many of these may not be usable in TALES, however, depending on the security policies of the template’s implementation.


jschinnerer (Nov 2, 2001 3:59 am; Comment #2) –

Relative Paths

Deceptively simple but not explicitly spelled out in the Path Expressions section. A very common filesystem command form doesn’t work in ZPT paths and confused me for a bit. Key phrase is this one:

Since every path must start with a variable name, …

so this works (to go up a node, then out another branch):


but this does not:


hathawsh (Apr 19, 2002 3:24 pm; Comment #3) –

This spec should describe the following path expression syntax:

<span tal:content="here/?myvar">

The ‘here/?myvar’ expression is evaluated by traversing from ‘here’ to the name given by the value associated with ‘myvar’. For example, if ‘myvar’ is set to “title”, ‘here/?myvar’ is equivalent to ‘here/title’.

bwarsaw (Jun 3, 2002 1:29 pm; Comment #4) –

  1. integer 0 is false
  2. integer > 0 is true

what about integers < 0? :)

chrisw (Jul 18, 2002 3:04 am; Comment #5) –
where is the ‘structure’ prefix documented? I reckon it should probably be in here…
From djmaze Sun Nov 8 17:45:59 -0500 2009

“path segments, may contain letters, digits, spaces, and the punctuation characters underscore, dash, period, comma, and tilde.” Which dash are you talking about? http://en.wikipedia.org/wiki/Dash

Or are you talking about Hyphen-minus (U+002D)?

From FredDrake Mon Nov 9 07:33:48 -0500 2009
The hyphen-minus, which is what most programmers refer to as the dash.