I have a function to replace the content of an ElementTree Element by
that of another one which works using Python 2 but not with Python 3.
I get an assertion error.  

It was suggested on the Python list that the problem is that in Python 3
slice assignments are done with __setitem__ rather than __setslice__ but
ElementTree has not been adapted to that -- hence the title given to
this bug.

The function that I use to demonstrate this (with a workaround solution)
is as follows:

def replace_element(elem, replacement):
    '''replace the content of an ElementTree Element by that of
another
       one.
    '''
    elem.clear()
    elem.text = replacement.text
    elem.tail = replacement.tail
    elem.tag = replacement.tag
    elem.attrib = replacement.attrib
    try:
        elem[:] = replacement[:]  # works with Python 2.5 but not 3.1
    except AssertionError:
        del elem[:]
        for child in replacement:
            elem.append(child)

I have included a small test file which demonstrate the behaviour.

Should be fixed with the upstream version, proposed on #6472.

Fixed in 2.7 with #6472. It should be ported to 3.x later.