Example Code to reproduce:

from xml.etree import ElementTree as etree
class xetree:
	cElement = etree.Element
	class Element(etree.Element):
		def __init__(self, tag, attrib=None):
			xetree.cElement.__init__(self, tag, attrib)

etree.Element = xetree.Element

e = etree.Element("test", {'foobar':'bar'})
e.text = "failure"
print(etree.tostring(e))
# will lack "failure"

So basic inheritance is broken.

Here's a simpler testcase:

import xml.etree.ElementTree as ET

class XElement(ET.Element):
    def __init__(self, tag, attrib={}):
        ET.Element.__init__(self, tag, attrib)

e = XElement('test')
e.text = 'failure'

print(ET.tostring(e))

New changeset 41a9d24d075e by Eli Bendersky in branch 'default':
Issue #14849: setup Element data members to be assignable in subclasses
http://hg.python.org/cpython/rev/41a9d24d075e