[lxml-dev] finding the line number of a parsed element

[Stefan Behnel]

Hi,

Stephan Richter wrote:
> I have recently reimplemented RML (Reportlab's XML format to generate PDFs) 
> using lxml. All works well.

Interesting. Any chance you could provide a link?


> Now, I would like to give my users some more information when an error occurs. 
> For a pure XML parsing error, everything is fine (though I found the failure 
> points hard to interpret at times). But what if the XML parses correctly, but 
> while working with the element tree an error occurs? In this case I would 
> like to tell the user not only the error message, but also the line/column 
> and filename of point of failure.

This sounds a lot like a problem you could try to solve with validation.


> Ideally I would have the filename, start row and start column of each element 
> available as part of the etree Element. I have tried to find this information 
> or hooks for it.unsuccessfully.

There is no API for it, but internally, we have this information for parsed
trees, at least the line number - note that exceptions contain the line number
already. So we could easily add a property "_line" to elements that returns
the line number at which the element was parsed (*if* it was parsed). I don't
like the fact so much that libxml2 puts a zero there if the node was created
by hand, but I assume that is not too much of a problem either.

I personally prefer "_line" over "line", as this only applies to parsed
elements, not all of them, so this is more of a half-working API.
Additionally, any additional attribute there goes off the list of children
accessible in objectify.

We could also consider adding an external utility module to provide helpers
like this that are not really worth poluting the API. Something like

lxml.tools.lineof(element)

But maybe that's just overkill...

Any comments?

Stefan