How to add a new Element to a musicXML tree in Python (elementtree)?

I am using Python to batch edit many musicXML files that currently look like this:

    <score-partwise>
    ...
      <attributes>
        <transpose>
          <diatonic>-5</diatonic>
          <chromatic>-9</chromatic>
          </transpose>
        </attributes>
    ...
      </score-partwise>

How can I add <octave-change>-1</octave-change> in <transpose></transpose>, as below?

    <score-partwise>
    ...
      <attributes>
        <transpose>
          <diatonic>-5</diatonic>
          <chromatic>-9</chromatic>
          <octave-change>-1</octave-change>
          </transpose>
        </attributes>
    ...
      </score-partwise>

I have attempted this:

import xml.etree.ElementTree as ET

attributes   = ET.Element("attributes")
attributes.append(ET.fromstring('<transpose><octave-change>-1</octave-change></transpose>'))

without success.

Any help is very much appreciated. Thank you.

Solution

Just find the element and append:

x = """<score-partwise>    
      <attributes>
        <transpose>
          <diatonic>-5</diatonic>
          <chromatic>-9</chromatic>
          </transpose>
        </attributes>    
      </score-partwise>"""

import xml.etree.ElementTree as et
xml = et.fromstring(x)

#
xml.find("attributes").append(et.fromstring('<transpose><octave-change>-1</octave-change></transpose>'))

print(et.tostring(xml))

Which gives you:

<score-partwise>
      <attributes>
        <transpose>
          <diatonic>-5</diatonic>
          <chromatic>-9</chromatic>
          </transpose>
        <transpose><octave-change>-1</octave-change></transpose></attributes>
</score-partwise>

That adds a new transpose element also, if you just want to append to the existing transpose element then select that.

import xml.etree.ElementTree  as et

xml = et.fromstring(x)


xml.find(".//attributes/transpose").append(et.fromstring('<octave-change>-1</octave-change>'))

print(et.tostring(xml))

Which gives you:

<score-partwise>
      <attributes>
        <transpose>
          <diatonic>-5</diatonic>
          <chromatic>-9</chromatic>
          <octave-change>-1</octave-change></transpose>
        </attributes>
</score-partwise>

You can also use SubElement which allows you to access the node:

xml = et.fromstring(x)

print(et.tostring(xml))
e = et.SubElement(xml.find(".//attributes/transpose"), "octave-change")
e.text = "-1"
e.tail= "\n"

If you want to formatting, you might find lxml to be a better option:

import lxml.etree as et

parser = et.XMLParser(remove_blank_text=True)
xml = et.parse("test.xml",parser)


xml.find(".//attributes/transpose").append(et.fromstring('<octave-change>-1</octave-change>'))
xml.write('test.xml', pretty_print=True)

Which will write:

<score-partwise>
  <attributes>
    <transpose>
      <diatonic>-5</diatonic>
      <chromatic>-9</chromatic>
      <octave-change>-1</octave-change>
    </transpose>
  </attributes>
</score-partwise>