xml.etree.ElementTree - Trouble setting xmlns = '...'

I must be missing something. I'm attempting to set up a google product feed, but am having a hard time registering the namespace.

Example:

Directions here: http://ift.tt/Vc7C3K

Trying to insert:

<rss version="2.0" 
xmlns:g="http://ift.tt/15FVsV8">

This is the code:

from xml.etree import ElementTree
from xml.etree.ElementTree import Element, SubElement, Comment, tostring

tree = ElementTree
tree.register_namespace('xmlns:g', 'http://ift.tt/15FVsV8')
top = tree.Element('top')
#... more code and stuff

After code is run, everything turns out fine, but we're missing the xmlns=

I find it easier to create XML docs in php, but I figured I'd give this a try. Where am I going wrong?

On that note, perhaps there is a more appropriate way to do this in python, not using etree?