id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc
91,naive use of concat in Path expresions causes obscure trace backs,robinbryce@…,cmlenz,"an example of naive use:
{{{
    <py:with vars=""var='greeting'"">
        <p py:match='.[p and @id=concat($var,""_7"")]' py:attrs='select(""@*"")'>
        matched
        </p>
        <p id=""greeting_7"">not matched</p>
    </py:with>

    <py:for each=""suffix in map(str, range(1,4))"">
    <p>$suffix</p>
    </py:for> 
}}}

And the tail of the traceback
{{{
  File ""/home/rbryce/devel-appliance/Genshi-0.3.6-py2.4.egg/genshi/path.py"", line 671, in __call__
    return u''.join(strings)
TypeError: sequence item 0: expected string or Unicode, NoneType found
}}}

The issue is that the ""var"" in @id=concat($var,""_7"") is lazily evaluated in the context of every candidate victim expresion. So, within the py:with, it's fine. Once the py:with scope closes, `type(var) is Undefined`. This kills `genshi.path.ConcatFunction.__call__`, as it assumes all expressions ultimately resolve to strings; but in the `$var` case its not a string and we get `TypeError`.

I think this should be some genshi equivelent of `RuntimeError`('This probably means you have not passed in the appropriate template global variables'). Certainly not `SyntaxError` or `TypeError`

I couldn't decide what I'd like to happen here: In the particular case of `concat` it '''only''' operates on strings and I was tempted to suggest some means of providing `$<eager>` (or perhaps `$<static>` is less gnomic, ) variables, which would get fully and permanently bound when the Function is constructed. Factoring this into the Path parser and template processing is far from minor surgery. 

Would it be a useful feature ? On reflection, probably not so much. I was essentially attempting to  templatize the template: Ie, procedurally generating a bunch of similar match templates. Given the way match templates are applied this is probably more than a little foolish (I've got a small set of 'well known divs' and a collection of 'similar' match templates that apply to each), and use of other XPath functions with a '''single''' match template are, I think, the right answer for me.

Posting this ticket in the hope of some free advice ;-)",defect,new,minor,,XPath support,devel,,,rbryce@…