<span tal:on-error "string: error message" > error message </span>
what is string: mean ? ....
     i mean namespace or  .....

Yes - "string:" is a namespace declaration, as would be "python:".

Strictly speaking, "string:" is not a namespace declaration in the
XML sense. Those can only occur before tags or attributes, not
inside the value of an attribute.

In the page template sense, "string:" is a TALES (Template Attribute
Language Expression Syntax).

From  http://www.plope.com/Books/2_7Edition/AppendixC.stx#1-10

Here is the basic definition of TALES syntax:

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

Here are some simple examples:

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.

Jean Jordaan
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