from lxml import etree

def add_child(root, field, value, optional=False):
  if not value and optional:  # if this is an optional field and no value is provided, skip writing the tag altogether
      return

  if not value:  # if the value is None, set it to an empty unicode object
      value = u''

  if isinstance(value, int):  # convert integers to string
      value = str(value)

  child = etree.SubElement(root, field)
  child.text = value
multipackid = "onethreethreeseven"
multipackroot = etree.Element("Multipack")
add_child(multipackroot, "id", multipackid)

string_io = StringIO.StringIO(
etree.tostring(multipackroot, pretty_print=True))
for thing in string_io:
  print thing