code: def _format_native_types(self, na_rep='', quoting=None, **kwargs):
          
          mask = isna(self)
          if not self.is_object() and not quoting:
              values = np.asarray(self).astype(str)
          else:
              values = np.array(self, dtype=object, copy=True)
  
          values[mask] = na_rep
          return values
docs: Actually format specific types of the index.