code: def to_sax(self, instance, dest):
          
          obj = self.__get__(instance, type(instance))
          if obj is None:
              return
          obj.unparse_to_sax(dest)
docs: Take the object associated with this descriptor on `instance` and
          serialize it as child into the given :class:`lxml.etree.Element`
          `parent`.
  
          If the object is :data:`None`, no content is generated.