def _add_line_str(target_str, line_str):
    """
    Add target line string for string concatenation.

    Parameters
    ----------
    target_str : str
        The string to be concatenated.
    line_str : str
        String of target line.

    Returns
    -------
    target_str : str
        Concatenated string.
    """
    if target_str != '':
        target_str += '\n'
    target_str += line_str
    return target_str
