This issue is only visible when the C accelerator of ElementTree is *not* used. It is the counterpart of the following issue on PyPy3: https://foss.heptapod.net/pypy/pypy/-/issues/3181

 >>> from xml.etree.ElementTree import Element
 >>> r = Element("root")
 >>> r.extend((Element(str(i)) for i in range(3)))
 >>> print(list(r))
 []

When using the C accelerator, the list is not empty, as expected. In the Python code, a check on the input empties the input iterator.

The fix is trivial (one-line change), so if you are interested I could open a PR, which would be my first, so a good occasion to go through the devguide ;)
I understand that since Python3.3 the C accelerator is used by default, so I would agree that this is not really a bug, and I can just fix it on PyPy side.

Your proposed fix LGTM. Please open a PR. Don't forget about test and NEWS entry.

Thanks for the report and the PR. I would argue that this can still be fixed in Py3.8, given the low impact (but not earlier). The behaviour is clearly wrong (in a subtle way) and most users won't notice it because they won't switch off the C accelerator module.

+1 for back porting, but check with the RM first.

Is there anything else I should be doing, in order to get this merged in time for 3.10? (the PR is "awaiting merge")
For the record, the equivalent PR for PyPy is now merged in the py3.7 branch: https://foss.heptapod.net/pypy/pypy/-/merge_requests/807.

Regarding the backporting, I understand from the devguide this is done automatically from GitHub labels (nice!), but it does not look like I have the rights to add those to my PR.

New changeset 51a85ddce8b336addcb61b96f04c9c5edef07296 by Alex Prengère in branch 'master':
bpo-43399: Fix ElementTree.extend not working on iterators (GH-24751)
https://github.com/python/cpython/commit/51a85ddce8b336addcb61b96f04c9c5edef07296

Pinging us was the only action needed. Thank you for your contribution Alex.

New changeset c1079cde2a7676892a9b98703903206b7d26ed1f by Miss Islington (bot) in branch '3.8':
bpo-43399: Fix ElementTree.extend not working on iterators (GH-24751)
https://github.com/python/cpython/commit/c1079cde2a7676892a9b98703903206b7d26ed1f

New changeset b500bd8e672d15c6dfa24568a3264fdc0f3e0c01 by Miss Islington (bot) in branch '3.9':
bpo-43399: Fix ElementTree.extend not working on iterators (GH-24751)
https://github.com/python/cpython/commit/b500bd8e672d15c6dfa24568a3264fdc0f3e0c01