Merge two xml files Python and also keep comments

Question

I'm trying to merge two xml files in python with the following code. That I found in another thread: Merge xml files with nested elements without external libraries

import sys
from xml.etree import ElementTree as et


class hashabledict(dict):
    def __hash__(self):
        return hash(tuple(sorted(self.items())))


class XMLCombiner(object):
    def __init__(self, filenames):
        assert len(filenames) > 0, 'No filenames!'
        # save all the roots, in order, to be processed later
        self.roots = [et.parse(f).getroot() for f in filenames]

    def combine(self):
      for r in self.roots[1:]:
        # combine each element with the first one, and update that
        self.combine_element(self.roots[0], r)
    # return the string representation
      return et.ElementTree(self.roots[0])

    def combine_element(self, one, other):
    """
    This function recursively updates either the text or the children
    of an element if another element is found in `one`, or adds it
    from `other` if not found.
    """
    # Create a mapping from tag name to element, as that's what we are fltering with
    mapping = {(el.tag, hashabledict(el.attrib)): el for el in one}
    for el in other:
        if len(el) == 0:
            # Not nested
            try:
                # Update the text
                mapping[(el.tag, hashabledict(el.attrib))].text = el.text
            except KeyError:
                # An element with this name is not in the mapping
                mapping[(el.tag, hashabledict(el.attrib))] = el
                # Add it
                one.append(el)
        else:
            try:
                # Recursively process the element, and update it in the same way
                self.combine_element(mapping[(el.tag, hashabledict(el.attrib))], el)
            except KeyError:
                # Not in the mapping
                mapping[(el.tag, hashabledict(el.attrib))] = el
                # Just add it
                one.append(el)

if __name__ == '__main__':

  r = XMLCombiner(sys.argv[1:-1]).combine()
  print '-'*20
  print et.tostring(r.getroot())
  r.write(sys.argv[-1], encoding="iso-8859-1", xml_declaration=True)

The code works perfectly for merging two xml files, however I would also like to merge the comments I have in the files. I'm new at this and don't know how to not just merge the xml but also the comments I have in the files.