hadoop configuration xml encode

I make a python script to deploy hadoop automatically. the xml config files, such as "core-site.xml", was modified & encoded by utf-8 with python script. But the hadoop can't read this xml file.

Here is my xml after modified by script:

<?xml version='1.0' encoding='utf-8'?>
<?xml-stylesheet type="text/xsl" href="configuration.xsl"?>
<!-- Put site-specific property overrides in this file. -->
<configuration>
    <property>
　　　　<name>fs.default.name</name>
　　　　<value>http://hdfsc31:9000</value>
　　</property>
</configuration>

Here is my python script:

import lxml.etree

def modify_core_site(namenode_hostname):
    doc = lxml.etree.parse("pkg/core-site.xml")
    root = doc.getroot()
    print dir(root)
    # print root.getchildren()
    for child in root.iter("property"):
        name = child.find("name").text
        if name == "fs.default.name":
            text = "hdfs://%s:9000" % namenode_hostname
            child.find("value").text = text
    s = lxml.etree.tostring(doc, encoding="utf-8", xml_declaration = True, pretty_print = True)
    with open("pkg/core-site.xml", "w") as f:
        f.write(s)

Where is the bug? Somebody help?