def set_tif_names(directory, file_names):
    """
    This function returns the directory + file_name
    of the inputs.

    In general, it is used to do mosaic with many
    geotiff files.

    :param directory:       String
    :param file_names:      String
    :return:                list
                            It returns a list with
                            Strings. Each string
                            has the merge of
                            the directory and file
                            name.
    """
    tif_names = []
    for i in file_names:
        tif_names.append(directory + i + '.tif')
    return tif_names