使用xpath与lxml.etree._ElementTree

Question

我有一个对象arptree，我试图定位一个ip地址的mac地址，但是失败了。

>>> arptree 
<lxml.etree._ElementTree object at 0x0000000004641688>

当我尝试使用以下xpath时，它将返回一个空列表

>>> arptree.xpath("descendant::mac-address[following-sibling::ip-address='10.69.119.150']")
[]

如果我修改xpath以排除"='10.69.119.150'"，它实际上返回一个元素列表。

>>> arptree.xpath("descendant::mac-address[following-sibling::ip-address]")                      

.

我可以使用for循环来访问内容。我确信ip地址10.69.119.150的mac地址就在那里。

for elt in arptree.iter():
    print elt.tag, elt.text

奇怪的是，如果我将xml输出复制并粘贴到xml文件中。然后使用：

from lxml import etree
tree = etree.parse(open('arp.xml'))
tree.xpath("descendant::mac-address[following-sibling::ip-address='10.69.119.150']")

它将返回ip地址的mac地址。

我在lxml包中使用Python2.7.9。有人能帮忙吗？

更新1:示例XML

<arp-table-information>
<arp-table-entry>
    <mac-address>00:a0:a5:76:1a:96</mac-address>
    <ip-address>10.69.119.130</ip-address>
    <hostname>10.69.119.130</hostname>
    <interface-name>vlan.49</interface-name>
    <arp-table-entry-flags>
        <none/>
    </arp-table-entry-flags>
</arp-table-entry>
<arp-table-entry>
    <mac-address>00:0f:bb:c6:26:3d</mac-address>
    <ip-address>10.69.119.150</ip-address>
    <hostname>10.69.119.150</hostname>
    <interface-name>vlan.55</interface-name>
    <arp-table-entry-flags>
        <none/>
    </arp-table-entry-flags>
</arp-table-entry>
</arp-table-information>

更新2:请忽略更新1

当我用

arptree = ex.device.rpc.get_arp_table_information().getroottree()
arptree.write('arptree.xml', pretty_print=True)

若要将ElementTree保存为xml，布局更改为

<arp-table-information style="normal">
<arp-table-entry>
<mac-address>
00:a0:a5:76:1a:96
</mac-address>
<ip-address>
10.69.119.130
</ip-address>
<hostname>
10.69.119.130
</hostname>
<interface-name>
vlan.49
</interface-name>
<arp-table-entry-flags>
<none/>
</arp-table-entry-flags>
</arp-table-entry>
<arp-table-entry>
<mac-address>
00:0f:bb:c6:26:3d
</mac-address>
<ip-address>
10.69.119.150
</ip-address>
<hostname>
10.69.119.150
</hostname>
<interface-name>
vlan.55
</interface-name>
<arp-table-entry-flags>
<none/>
</arp-table-entry-flags>
</arp-table-entry>

也许这就是下面的代码不能工作的原因？

arptree.xpath("descendant::mac-address[following-sibling::ip-address='10.69.119.150']")

基于这个xml文件，有人能帮忙吗？

Answer 1

在第二个XML中，在IP地址值前后有新的行字符。您可以使用normalize-space()函数来修复它：

descendant::mac-address[following-sibling::ip-address[normalize-space()='10.69.119.150']]

演示的工作示例:

from lxml import etree

xml = """<arp-table-information style="normal">
<arp-table-entry>
<mac-address>
00:a0:a5:76:1a:96
</mac-address>
<ip-address>
10.69.119.130
</ip-address>
<hostname>
10.69.119.130
</hostname>
<interface-name>
vlan.49
</interface-name>
<arp-table-entry-flags>
<none/>
</arp-table-entry-flags>
</arp-table-entry>
<arp-table-entry>
<mac-address>
00:0f:bb:c6:26:3d
</mac-address>
<ip-address>
10.69.119.150
</ip-address>
<hostname>
10.69.119.150
</hostname>
<interface-name>
vlan.55
</interface-name>
<arp-table-entry-flags>
<none/>
</arp-table-entry-flags>
</arp-table-entry>
</arp-table-information>"""

root = etree.fromstring(xml)
result = root.xpath("descendant::mac-address[following-sibling::ip-address[normalize-space()='10.69.119.150']]")
for r in result:
    print(etree.tostring(r))

输出:

<mac-address>
00:0f:bb:c6:26:3d
</mac-address>