code: def _append_national_number(self, national_number):
          
          prefix_before_nn_len = len(self._prefix_before_national_number)
          if (self._should_add_space_after_national_prefix and prefix_before_nn_len > 0 and
              self._prefix_before_national_number[-1] != _SEPARATOR_BEFORE_NATIONAL_NUMBER):
              # We want to add a space after the national prefix if the national
              # prefix formatting rule indicates that this would normally be
              # done, with the exception of the case where we already appended a
              # space because the NDD was surprisingly long.
              return self._prefix_before_national_number + _SEPARATOR_BEFORE_NATIONAL_NUMBER + national_number
          else:
              return self._prefix_before_national_number + national_number
docs: Combines the national number with any prefix (IDD/+ and country
          code or national prefix) that was collected. A space will be inserted
          between them if the current formatting template indicates this to be
          suitable.