docs: def _validate_charset(data, charset):
          """"Validate that the charset is correct and throw an error if it isn't."""
          if len(charset) > 1:
              charset_data_length = 0
              for symbol_charset in charset:
                  if symbol_charset not in ('A', 'B', 'C'):
                      raise Code128.CharsetError
                  charset_data_length += 2 if symbol_charset is 'C' else 1
              if charset_data_length != len(data):
                  raise Code128.CharsetLengthError
          elif len(charset) == 1:
              if charset not in ('A', 'B', 'C'):
                  raise Code128.CharsetError
          elif charset is not None:
              raise Code128.CharsetError