etree.cleanup_namespaces does not remove a wrapping namespace in an unwrapped subtree

[Sebastian Albert]

This may either be a bug or a misunderstanding on my side.

I have an XML document with a wrapper namespace, and I am extracting the payload from a subtree. I want to get rid of the wrapper namespace, but using etree.cleanup_namespaces does not yield the expected result.

Here is a short example REPL session:

>>> from lxml import etree
>>> tree = etree.fromstring('<wrap:moep xmlns:wrap="http://example.com/wrap"><foo xmlns="http://example.com/content"><bar /></foo></wrap:moep>')
>>> tree
<Element {http://example.com/wrap}moep at 0x7f246363aec0>
>>> etree.tostring(tree)
b'<wrap:moep xmlns:wrap="http://example.com/wrap"><foo xmlns="http://example.com/content"><bar/></foo></wrap:moep>'
>>> foo = tree.find('{http://example.com/content}foo')
>>> etree.tostring(foo)
b'<foo xmlns="http://example.com/content" xmlns:wrap="http://example.com/wrap"><bar/></foo>'
>>> etree.cleanup_namespaces(foo)
>>> etree.tostring(foo)
b'<foo xmlns="http://example.com/content" xmlns:wrap="http://example.com/wrap"><bar/></foo>'

I would expect the xmlns:wrap to have been removed. Am I missing something, or is this a bug?

Python : sys.version_info(major=3, minor=8, micro=13, releaselevel='final', serial=0)
lxml.etree : (4, 9, 1, 0)
libxml used : (2, 9, 14)
libxml compiled : (2, 9, 14)
libxslt used : (1, 1, 35)
libxslt compiled : (1, 1, 35)