TALES Path expressions


Path expression syntax:

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


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 (built-in variable or a user defined variable), and the remaining strings, the path segments, may contain letters, digits, spaces, and the punctuation characters underscore, dash, period, comma, and tilde.

For example:

      here/some-file 2001_02.html.tar.gz/foo
      root/to/branch | default
      request/name | string:Anonymous Coward

When a path expression is evaluated, Zope attempts to traverse the path, from left to right, until it succeeds or runs out of paths segments. 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. Subobjects are located according to standard Zope traversal rules (via getattr, getitem, or traversal hooks).

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 template, it is called.

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

The expression in a series of paths seperated by vertical bars can be any TALES expression. For example, request/name | string:Anonymous Coward. This is useful chiefly for providing default values such as strings and numbers which are not expressable as path expressions.

If no path is given the result is nothing.

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. See the TALES overview for a list of built-in variables. Since variable names are looked up first in locals, then in globals, then in this list, these names act just like built-ins in Python; They are always available, but they can be shadowed by a global or local variable declaration. You can always access the built-in names explicitly by prefixing them with CONTEXTS. (e.g. CONTEXTS/root, CONTEXTS/nothing, etc).


Inserting a cookie variable or a property:

      <span tal:replace="request/cookies/pref | here/pref">

Inserting the user name:

      <p tal:content="user/getUserName">
        User name