[Tutor] ElementTree, iterable container, depth of elements

[street.sweeper at mailworks.org]

I'm trying to sort the order of elements in an xml file, mostly
to make visual inspection/comparison easier.  The example xml and
code on http://effbot.org/zone/element-sort.htm get me almost
what I need, but the xml I'm working with has the element I'm
trying to sort on one level deeper.


That page's example xml:

<phonebook>
  <entries>
    <entry>
      <name>Ned</name>
      <number>555-8904</number>
    </entry>
    <entry>
      <name>John</name>
      <number>555-5782</number>
    </entry>
    <entry>
      <name>Julius</name>
      <number>555-3642</number>
    </entry>
  </entries>
</phonebook>


And that page's last example of code:

  import xml.etree.ElementTree as ET
  tree = ET.parse("data.xml")
  def getkey(elem):
    return elem.findtext("number")
  container = tree.find("entries")
  container[:] = sorted(container,key=getkey)
  tree.write("new-data.xml")

I used the interactive shell to experiment a bit with that,
and I can see that 'container' in

  container = tree.find("entries")

is iterable, using

  for a in container:
    print(a)

However, the xml I'm working with looks something like this:

<root>
  <main>
    <diary>
      <entry>
        <Date>20140325</Date>
        <appointment>dentist</appointment>
      </entry>
      <entry>
        <Date>20140324</Date>
        <appointment>barber</appointment>
      </entry>
    </diary>
  </main>
</root>


What I'd like to do is rearrange the <entry> elements within
<diary> based on the <Date> element.  If I remove the <root>
level, this will work, but I'm interested in getting the code to
work without editing the file.

I look for "Date" and "diary" rather than "number" and "entries"
but when I try to process the file as-is, I get an error like


Traceback (most recent call last):
  File "./xmlSort.py", line 16, in <module>
    container[:] = sorted(container, key=getkey)
TypeError: 'NoneType' object is not iterable


"container[:] = sorted(container, key=getkey)" confuses me,
particularly because I don't see how the elem parameter is passed
to the getkey function.

I know if I do

  root = tree.getroot()

(from the python.org ElementTree docs) it is possible to step
down through the levels of root with root[0], root[0][0], etc,
and it seems to be possible to iterate with

  for i in root[0][0]:
    print(i)

but trying to work root[0][0] into the code has not worked,
and tree[0] is not possible.

How can I get this code to do its work one level down in the xml?

Thanks