directory_name = "C:\apps"
file_name = "web.config"

xpath_identifier = '/configuration/applicationSettings/Things/setting[@name="CorsTrustedOrigins"]'

#contents of the xpath node for reference:
block = '''
      <setting name="Things" serializeAs="Xml">
    <value>
      <ArrayOfString>
        <string>http://localhost:51363</string>
        <string>http://localhost:3333</string>
      </ArrayOfString>
    </value>
  </setting>
  '''

file_full_path = os.path.join(directory_name, file_name)
tree = etree.parse(file_full_path)
root = tree.getroot()

etree.tostring(root)


xpath_identifier = str(xpath_identifier)

value = root.xpath(xpath_identifier)

#This successfully prints the element I'm after, so I'm sure my xpath is good:
etree.tostring(value[0])

#This is the new xml element I want to replace the current xpath'ed element with:
newxml = '''
<setting name="CorsTrustedOrigins" serializeAs="Xml">
        <value>
          <ArrayOfString>
            <string>http://maddafakka</string>
          </ArrayOfString>
        </value>
      </setting>
'''

newtree = etree.fromstring(newxml)

#I've tried this:
value[0].getparent().replace(value[0], newtree)

#and this
value[0] = newtree

#The value of value[0] gets updated, but the "root document" does not.