TALES Overview

  The *Template Attribute Language Expression Syntax* (TALES) standard
  describes expressions that supply "TAL":tal.stx and
  "METAL":metal.stx 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":tales-path.stx expressions - locate a value by its path.

    * "exists":tales-exists.stx expressions - test whether a path is valid.

    * "nocall":tales-nocall.stx expressions - locate an object by its path.

    * "not":tales-not.stx expressions - negate an expression
  
    * "string":tales-string.stx expressions - format a string

    * "python":tales-python.stx 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":tal-repeat.stx 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":tal.stx

    "METAL Overview":metal.stx

    "exists":tales-exists.stx expressions

    "nocall":tales-nocall.stx expressions

    "not":tales-not.stx expressions
  
    "string":tales-string.stx expressions

    "path":tales-path.stx expressions

    "python":tales-python.stx expressions