def _read_xml_file(self):
        """
        read metadata from an xml file.

        :return: the root element of the xml
        :rtype: ElementTree.Element
        """
        # this raises a IOError if the file doesn't exist
        root = ElementTree.parse(self.xml_uri)
        root.getroot()
        return root
read metadata from an xml file.

        :return: the root element of the xml
        :rtype: ElementTree.Element