code: def _write_attribute_categorical(series, fp):
      
      if is_categorical_dtype(series.dtype):
          categories = series.cat.categories
          string_values = _check_str_array(categories)
      else:
          categories = series.dropna().unique()
          string_values = sorted(_check_str_array(categories), key=lambda x: x.strip('"'))
  
      values = ",".join(string_values)
      fp.write("{")
      fp.write(values)
      fp.write("}")
docs: Write categories of a categorical/nominal attribute