def sizes(s):
    s['size_kb'] = locale.format('%.1f', s['size'] / 1024.0, grouping=True
        ) + ' KB'
s['size_mb'] = locale.format('%.1f', s['size'] / 1024.0 ** 2, grouping=True
    ) + ' MB'
s['size_gb'] = locale.format('%.1f', s['size'] / 1024.0 ** 3, grouping=True
    ) + ' GB'
return s
df_test = df_test.append(rows_list)
df_test = df_test.apply(sizes, axis=1)
Return multiple columns from apply pandas