def mt_excel_files(store, case_obj, temp_excel_dir):
    """Collect MT variants and format line of a MT variant report
    to be exported in excel format

    Args:
        store(adapter.MongoAdapter)
        case_obj(models.Case)
        temp_excel_dir(os.Path): folder where the temp excel files are written to

    Returns:
        written_files(int): the number of files written to temp_excel_dir

    """
    today = datetime.datetime.now().strftime('%Y-%m-%d')
    samples = case_obj.get('individuals')

    query = {'chrom':'MT'}
    mt_variants = list(store.variants(case_id=case_obj['_id'], query=query, nr_of_variants= -1, sort_key='position'))

    written_files = 0
    for sample in samples:
        sample_id = sample['individual_id']
        sample_lines = export_mt_variants(variants=mt_variants, sample_id=sample_id)

        # set up document name
        document_name = '.'.join([case_obj['display_name'], sample_id, today]) + '.xlsx'
        workbook = Workbook(os.path.join(temp_excel_dir,document_name))
        Report_Sheet = workbook.add_worksheet()

        # Write the column header
        row = 0
        for col,field in enumerate(MT_EXPORT_HEADER):
            Report_Sheet.write(row,col,field)

        # Write variant lines, after header (start at line 1)
        for row, line in enumerate(sample_lines,1): # each line becomes a row in the document
            for col, field in enumerate(line): # each field in line becomes a cell
                Report_Sheet.write(row,col,field)
        workbook.close()

        if os.path.exists(os.path.join(temp_excel_dir,document_name)):
            written_files += 1

    return written_files
Collect MT variants and format line of a MT variant report
    to be exported in excel format

    Args:
        store(adapter.MongoAdapter)
        case_obj(models.Case)
        temp_excel_dir(os.Path): folder where the temp excel files are written to

    Returns:
        written_files(int): the number of files written to temp_excel_dir