def fname(file_name, extension):
    """Check a filename, adding an extension if not already specified.

    Parameters
    ----------
    file_name : str
        String that specifies a file name.
    extension : str
        String of the extension (without a period) to be added if one isn't already present.

    Outputs
    -------
    file_name : str
        String that specifies a file name.
    """
    if len(file_name.split('.')) == 1:
        file_name = file_name + '.' + extension
    return file_name
