Tail handling in addnext

[Jonathan Hofinger]

Downstream issue: https://gitlab.com/inkscape/extensions/-/issues/480

If elem has a tail, calling elem.addnext(elem2) inserts elem2 between elem and its tail, so this removes
elem's tail and moves it to elem2.

lxml decided that the tail belongs to the preceding element, so this relationship should stay intact over an addnext call. We're also not the first ones running into this issue, see https://github.com/lxml/lxml/blob/ea954da3c87bd8f6874f6bf4203e2ef5269ea383/TODO.txt#L33 or https://stackoverflow.com/questions/23282241/lxml-difference-between-element-addnext-and-insert-in-handling-tail.

Example code:

from lxml import etree
xml = etree.fromstring("<g><path/>tail</g>")
path = xml[0]
print(f"Tail of the first element: {path.tail}")
path.addnext(etree.Element("path"))
print(f"Tail of the first element after inserting: {path.tail}")
print(f"Full tree: {etree.tostring(xml)}")

Result:

Tail of the first element: tail
Tail of the first element after inserting: None
Full tree: b'<g><path/><path/>tail</g>'

Python : sys.version_info(major=3, minor=7, micro=8, releaselevel='final', serial=0)
lxml.etree : (4, 5, 2, 0)
libxml used : (2, 9, 5)
libxml compiled : (2, 9, 5)
libxslt used : (1, 1, 30)
libxslt compiled : (1, 1, 30)

[scoder (scoder)]

Right. This is a bug. This case was previously not covered by the test suite. I'll fix it in lxml 5.0. Thanks for the report.

[scoder (scoder)]

Fixed in https://github.com/lxml/lxml/commit/cc5ddbb1576d2844262a9d520867608aec956de9