'Return partname index as integer for tuple partname or None for
singleton partname, e.g. ``21`` for ``\'/ppt/slides/slide21.xml\'`` and
|None| for ``\'/ppt/presentation.xml\'``.'

@property
def idx(self):

    filename = self.filename
    if (not filename):
        return None
    name_part = posixpath.splitext(filename)[0]
    match = self._filename_re.match(name_part)
    if (match is None):
        return None
    if match.group(2):
        return int(match.group(2))
    return None
