code: def _set_fallback(elements, src_field, fallback, dest_field=None):
          
  
          if dest_field is None:
              dest_field = src_field
          if isinstance(fallback, six.string_types):
              fallback = elements[fallback]
  
          attrs = elements[src_field]
          elements[dest_field] = (
              attrs[0] if attrs[0] is not None else fallback[0],
              attrs[1] if attrs[1] is not None else fallback[1],
              attrs[2] if attrs[2] is not None else fallback[2])
docs: Helper function used to set the fallback attributes of an element when
          they are defined by the configuration as "None" or "-".