Variables Expansion In Paths


TALES expression syntax provides variable path expansion in strings using the ${path/to/variable} syntax. However, this syntax is not allowed when constructing paths. (While this behavior can be simulated using python: strings, it is not intuitive why variable expansion should work in one case but not in another.)

For example, this works currently:

<li tal:repeat="item here/queryIds">
  <a tal:attributes="href string:here/subfolder/${item/id}">Link</a>

whereas this does not:

<li tal:repeat="item here/queryIds">
  <span tal:content="path:here/subfolder/${item/id}/method" />


Allow for variable path expansion to occur when constructing paths. The most flexible way to do this might be to simply treat paths as strings, perform any expansions, and then traverse the resulting expression. The new syntax for path would be expressed as:

PathExpr  ::= Path [ '|' Path ]*
Path      ::= string_expression

where string_expression is already defined in TAL Reference as:

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

This would allow nested expansions:

<span tal:replace="path:here/${folder${item/id}}/method" />

as well as allowing the initial path variable to be, well, a variable:

<span tal:replace="${where}/object" />

Although if deemed undesirable, this usage could be disallowed while still allowing path expansion generally.

Rationale for Rejection

(Explanation from JimFulton.)

First of all, I see no justification for the generality of full string interpolation, with interpolated values treated as sub-expressions. The example in the proposal doesn’t require this generality. No one has given any convincing use cases why this (full string interpolation) is needed.

I do support making the current undocumented syntax official. The current syntax allows individual path segments to be retrieved from variables, as in:


Characters in the path segment variable are given no special meaning. This is especially useful if the object being traversed is a mapping object. A mapping object may have keys that contain special characters, like ‘/’s and ‘|’s. It’s important that these characters be used as data. Otherwise the mapping items would be inaccessible.