lxml_create_tag.py

# make a new, empty tag
>>> new_tag = etree.Element("extent")

# add some text
>>> new_tag.text = "25 embarrassing photos"

# add an attribute if you want any
>>> new_tag.attrib["encodinganalog"] = "300"

# insert the new tag into the master ead tree
# we'll use the parent element from last time
>>> parent.append(new_tag)
>>> print(etree.tostring(parent))
'''
<physdesc altrender="whole">
    <extent encodinganalog="300">25 embarrassing photos</extent>
</physdesc>
'''