Add element.sourceline (lxml)

[SimonSapin]

Elements in lxml have a sourceline attribute, but it is always None in trees parsed with html5lib.

html5lib also appears to keep track of source line numbers for error messages.

I’d like to connect the two so that WeasyPrint can do its own error reporting based on line numbers of HTML elements. It seems like the attribute should be set in etree_lxml.py, but I don’t know how to access the input stream from there.

Suggestions?

[SimonSapin]

This could be opt-in if it degrades performance noticeably.

[gsnedders]

See also #87.

[tahajahangir]

Previously was requested in http://code.google.com/p/html5lib/issues/detail?id=213

[SimonSapin]

In http://code.google.com/p/html5lib/issues/detail?id=213 :

Off-hand, I believe there is no API to set sourceline, so we cannot (even if we had the data) expose it in such a way.

I believe that the API to set sourceline is:

element.sourceline = integer_value