ElementTree's Element substitution in Python 3

[Terry Reedy]

André wrote:
> 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.  The function 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
>     elem[:] = replacement[:]
> 
> The last line is problematic.  For example, if I do the following
> program with Python2.5
> ###
> from xml.etree import ElementTree as et
> 
> a = et.Element('a')
> b = et.SubElement(a, 'b')
> c = et.Element('c')
> 
> a[:] = c[:]
> ###
> nothing of note happens - however, doing the same with Python 3.1, I
> get the following traceback:
> 
> Traceback (most recent call last):
>   File "test.py", line 7, in <module>
>     a[:] = c[:]
>   File "/usr/local/py3.1/lib/python3.1/xml/etree/ElementTree.py", line
> 210, in __setitem__
>     assert iselement(element)
> AssertionError
> 
> ======
> I would gladly welcome any suggestion for writing a replace_element()
> function that works with Python 3.1

My guess is that you found a subtle bug in the 3.1 version of 
ElementTree, perhap a result of conversion. I would take a look at the 
noted line 210 to see whether it is 'a' or 'c' that is not passing as an 
element. Assuming that it is 'c', I would look at __getitem__ to see why 
not. Is [:] special-cased? Compare the codes for 2.5 and 3.1. Assuming 
it still looks like a bug, report it on the tracker using your minimal 
example, leaving out your function.

tjr