How to update XML file with lxml

Here you go, get the root of the tree, append your new element, save the tree as a string to a file:

from lxml import etree

tree = etree.parse('books.xml')

new_entry = etree.fromstring('''<book category="web" cover="paperback">
<title lang="en">Learning XML 2</title>
<author>Erik Ray</author>
<year>2006</year>
<price>49.95</price>
</book>''')

root = tree.getroot()

root.append(new_entry)

f = open('books-mod.xml', 'wb')
f.write(etree.tostring(root, pretty_print=True))
f.close()