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xml.etree.ElementTree.Element's append method doesn't support iterator/sequence parameters as it's supposed to, for the version 1.3.0.
Python 2.7.3 (default, Sep 14 2012, 09:52:31) 
[GCC 4.2.1 Compatible Apple Clang 4.0 ((tags/Apple/clang-421.0.60))] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import xml.etree.ElementTree as et
>>> et.VERSION
'1.3.0'
>>> root = et.Element('root')
>>> sublist = [et.Element('sub'), et.Element('sub')]
>>> root.append(sublist)
>>> et.tostring(root)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 1127, in tostring
 ElementTree(element).write(file, encoding, method=method)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 818, in write
 self._root, encoding, default_namespace
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 878, in _namespaces
 for elem in iterate():
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 477, in iter
 for e in e.iter(tag):
AttributeError: 'list' object has no attribute 'iter'
>>> root = et.Element('root')
>>> root.append(iter(sublist))
>>> et.tostring(root)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 1127, in tostring
 ElementTree(element).write(file, encoding, method=method)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 818, in write
 self._root, encoding, default_namespace
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 878, in _namespaces
 for elem in iterate():
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 477, in iter
 for e in e.iter(tag):
AttributeError: 'listiterator' object has no attribute 'iter'

(unassigning as this is not a mac-specific issue)
BTW. Is this really a bug, the documentation says that append appends a single element <http://docs.python.org/library/xml.etree.elementtree.html#xml.etree.ElementTree.Element.append>:
<quote>
append(subelement)
Adds the element subelement to the end of this elements internal list of subelements.
</quote>

well, i've just followed the source code regardless to documentation so you may be right about appending a single element. (kind of newbie around here.)
but then, append method is misbehaving about asserting the parameter, isn't it?

Use the extend method to add multiple elements. Both the source and documentation indicate that 'append' is used for appending a single item and 'extend' for appending multiple items (just like with list).
IMHO this is not a bug.
As an aside: when you import xml.etree.cElementTree you get a faster implementation of the same interface, and this (C-based) implementation does validate arguments.

Closing, since this isn't a bug and append's behavior is properly documented.
Regarding the error message, yes it could probably be better but you would need to enable input validation for that. Since Python is duck typed, often when arguments are not validated you get less-than-good error messages if things go wrong. In this case, append expect something Element-like that would have an "iter" method, and complains when that's not found.
In 3.3 this whole thing was improved by always validating arguments in append/extend etc. and raising a TypeError when something is wrong. I don't think there's good enough reason to change this in 2.7