This patch provides to the 'current' position expat api
via cElementTree.XMLTreeBuilder.

pyexpat already exposes this via the attributes
CurrentLineNumber, CurrentColumnNumber and
CurrentByteIndex. In order for cElementTree to do the
same, the following expat functions are added to those
exported via the pyexpat capi:
 * XML_CurrentLineNumber
 * XML_CurrentColumnNumber
 * XML_CurrentByteIndex

Then, in _elementtree.c, the xmlparser_getattr is made
to mirror the provisions made in pyexpat. A trivial
test is added to test_etree_c.py to cover the three new
attributes.

This patch was motivated by a discussion[1] on the
turbogears list that referenced two xml templating
libraries Kid & Markup. One of the benefits of the
latter is the ability to report line numbers for
template source errors.

If this patch is applied it should be possible for Kid,
and anything else that uses cElementTree for
xmlparsing, to do the same.

[1]
http://groups.google.com/group/turbogears/browse_frm/thread/1ed59ed984fc8bbe/1f06eb00e83f93d9#1f06eb00e83f93d9

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Fredrik, do you want etree patches here?

In the upstream 1.0.6, the ParseError exception has a position attribute
that contains a (line, column) tuple.

Which will be synced into core at some point, I guess?

> In the upstream 1.0.6, the ParseError exception has a position attribute
> that contains a (line, column) tuple.

That's fine for errors in the xml domain. Its not enough if I'm
reporting errors in the application domain - where I wont have a handy
exception to interrogate. What's the problem with just exposing the api
and requiring users who need it to use the streaming parser ?

Personally, I would have cached these details as a properties of the
_ElementInterface implementation for precisely this use case.

ElementTree fixed in 2.7. See #6472.