updating XML attribute value in python

import xml.etree.ElementTree as ET

tree = ET.parse('data')
for node in tree.getiterator():
    if int(node.attrib.get('age', 0)) > 21:
        node.attrib['alcohol'] = 'yes'
root = tree.getroot()
ET.register_namespace("", "XYZ")
print(ET.tostring(root))

yields

<root xmlns="XYZ" usingPalette="">

<grandParent hostName="XYZ">
<parent>
        <child name="JohnsDad">
            <grandChildren age="22" alcohol="yes" name="John" sex="male" />
        </child>
        <child name="PaulasDad">
            <grandChildren age="15" alcoho="no" name="Paula" sex="female" />
        </child>
</parent>
</grandParent>     
</root>

By the way, since the XML uses the namespace "XYZ", you must specify the namespace in your XPath:

for node in tree.findall('.//{XYZ}grandParent'):
    print node

That will return the grandParent element, but since you want to inspect all subnodes, I think using getiterator is easier here.

To preserve comments while using xml.etree.ElementTree you could use the custom parser Fredrik Lundh shows here:

import xml.etree.ElementTree as ET


class PIParser(ET.XMLTreeBuilder):
    """
    http://effbot.org/zone/element-pi.htm
    """
    def __init__(self):
        ET.XMLTreeBuilder.__init__(self)
        # assumes ElementTree 1.2.X
        self._parser.CommentHandler = self.handle_comment
        self._parser.ProcessingInstructionHandler = self.handle_pi
        self._target.start("document", {})

    def close(self):
        self._target.end("document")
        return ET.XMLTreeBuilder.close(self)

    def handle_comment(self, data):
        self._target.start(ET.Comment, {})
        self._target.data(data)
        self._target.end(ET.Comment)

    def handle_pi(self, target, data):
        self._target.start(ET.PI, {})
        self._target.data(target + " " + data)
        self._target.end(ET.PI)


tree = ET.parse('data', PIParser())

Note that if you install lxml, you could instead use:

import lxml.etree as ET
parser = ET.XMLParser(remove_comments=False)
tree = etree.parse('data', parser=parser)