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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