Example:

# mre.py
from xml.etree import ElementTree

XML = "<a>foo<!-- comment -->bar</a>"

a = ElementTree.fromstring(XML)
print(list(a.itertext()))

# Testing 3.7.3 vs. 3.8.0b1; macOS

… ~> python3.7 mre.py
['foobar']
… ~> python3.8 mre.py
['bar']

Bisecting gives me commit 43851a202c (issue36673) before which "foobar" was returned and after the commit "bar" is returned.

Just to add the Python implementation seems to return "foobar" on commenting the C accelerators imports. So I guess it's a problem with the C implementation in the commit being different from Python implementation.

I think it might be this call that strikes here:

https://github.com/python/cpython/commit/43851a202c#diff-f3b827d6e1d5c270ab42bc2c0523c1d2R2842

treebuilder_flush_data() is not made for concatenating text, it simply replaces it. If both text parts come separately, and the comment between the two is discarded, then the last one overwrites the first one.

Yes that does look suspicious!

I'm working on a patch. It's not entirely trivial, so it might take a couple of days.

New changeset c6cb4cdd21c0c3a09b0617dbfaa7053d3bfa6def by Stefan Behnel in branch 'master':
bpo-37399: Correctly attach tail text to the last element/comment/pi (GH-14856)
https://github.com/python/cpython/commit/c6cb4cdd21c0c3a09b0617dbfaa7053d3bfa6def

New changeset bb697899aa65d90488af1950ac7cceeb3877d409 by Stefan Behnel in branch '3.8':
[3.8] bpo-37399: Correctly attach tail text to the last element/comment/pi (GH-14856) (GH-14936)
https://github.com/python/cpython/commit/bb697899aa65d90488af1950ac7cceeb3877d409