def _newline_end_of_file(relative_path, contents, linter_options):
    r"""Check that every file ends with a single \n."""
    del relative_path
    del linter_options
    last_line = len(contents) - 1
    if not len(contents) or not contents[last_line].endswith("\n"):
        if len(contents):
            last_line_replacement = contents[last_line] + "\n"
        else:
            last_line_replacement = "\n"
        return LinterFailure(r"No \n at end of file",
                             last_line + 1,
                             last_line_replacement)