ElementTree.tostring（）：UnicodeDecodeError：

Question

我从这个简单的代码段中得到了一个例外：

# -*- coding: utf-8 -*-
from __future__ import absolute_import, division, print_function, unicode_literals

import xml.etree.cElementTree as ET
tables = ET.Element('tables')
table = ET.SubElement(tables, 'table', name='my_table')
head = dict()
tr_th = ET.SubElement(table, 'tr')
head = sorted(head.keys())
ET.SubElement(tr_th, 'th').text = 'ü'
tr_td = ET.SubElement(table, 'tr')
ET.SubElement(tr_td, 'td').text = 'ä'
ET.tostring(tables, encoding='utf8', method='xml')

例外：

user@host:~$ python ~/src/test_etree.py 
Traceback (most recent call last):
  File "/home/modlink_cok_d/src/test_etree.py", line 13, in <module>
    ET.tostring(tables, encoding='utf8', method='xml')
  File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1127, in tostring
    return "".join(data)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 0: 
                    ordinal not in range(128)

Python 2.7.12

我做错了什么？