I could be wrong, but I <b>think</b> because of pass-by-reference in python this will result in modifying <code>backup['default'].persistent value</code>.  This is hidden side-effect about python dictionaries that gets me in trouble a lot also :S 

Good to double-check.  If it is changing the backup, then better work on a <code>default_xml = backup['default'].copy()</code> to be safe.