def StringDecoder(field_number, is_repeated, is_packed, key, new_default):
  """Returns a decoder for a string field."""

  local_DecodeVarint = _DecodeVarint
  local_unicode = six.text_type

  def _ConvertToUnicode(byte_str):
    try:
      return local_unicode(byte_str, 'utf-8')
    except UnicodeDecodeError as e:
      # add more information to the error message and re-raise it.
      e.reason = '%s in field: %s' % (e, key.full_name)
      raise

  assert not is_packed
  if is_repeated:
    tag_bytes = encoder.TagBytes(field_number,
                                 wire_format.WIRETYPE_LENGTH_DELIMITED)
    tag_len = len(tag_bytes)
    def DecodeRepeatedField(buffer, pos, end, message, field_dict):
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      while 1:
        (size, pos) = local_DecodeVarint(buffer, pos)
        new_pos = pos + size
        if new_pos > end:
          raise _DecodeError('Truncated string.')
        value.append(_ConvertToUnicode(buffer[pos:new_pos]))
        # Predict that the next tag is another copy of the same repeated field.
        pos = new_pos + tag_len
        if buffer[new_pos:pos] != tag_bytes or new_pos == end:
          # Prediction failed.  Return.
          return new_pos
    return DecodeRepeatedField
  else:
    def DecodeField(buffer, pos, end, message, field_dict):
      (size, pos) = local_DecodeVarint(buffer, pos)
      new_pos = pos + size
      if new_pos > end:
        raise _DecodeError('Truncated string.')
      field_dict[key] = _ConvertToUnicode(buffer[pos:new_pos])
      return new_pos
    return DecodeField
Returns a decoder for a string field.