如何用Python和XPath仅获取显式定义默认命名空间的XML元素

我用Python的lxml解析XML字符串，需要编写XPath表达式，只获取那些**自身显式定义了默认命名空间（无前缀的xmlns属性）**的元素。

当前示例代码：

from lxml import etree

xml = '''
<TELCAL_DATAMANAGER_BDNT_1
    xmlns="urn:schemas-cosylab-com:BulkDataNTReceiver:1.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:schemas-cosylab-com:BulkDataNTReceiver:1.0 BulkDataNTReceiver.xsd">
    <ReceiverStream Name="InterferometricStream1" participantPerStream="true">
        <ReceiverFlow Name="FullSpectralFlow" cbReceiveAvgProcessTimeoutSec="0.0018" multicastAddress="225.3.2.100"></ReceiverFlow>
        <ReceiverFlow Name="ChannelAveragesFlow" multicastAddress="225.3.2.200"></ReceiverFlow>
        <ReceiverFlow Name="WVRFlow" multicastAddress="225.3.2.30"></ReceiverFlow>
    </ReceiverStream>
    <ReceiverStream Name="InterferometricStream2" participantPerStream="true">
        <ReceiverFlow Name="FullSpectralFlow" cbReceiveAvgProcessTimeoutSec="0.0018" multicastAddress="225.3.2.100"></ReceiverFlow>
        <ReceiverFlow Name="ChannelAveragesFlow" multicastAddress="225.3.2.200"></ReceiverFlow>
        <ReceiverFlow Name="WVRFlow" multicastAddress="225.3.2.30"></ReceiverFlow>
    </ReceiverStream>
    <ReceiverStream Name="InterferometricStream3" participantPerStream="true">
        <ReceiverFlow Name="FullSpectralFlow" cbReceiveAvgProcessTimeoutSec="0.0018" multicastAddress="225.3.2.100"></ReceiverFlow>
        <ReceiverFlow Name="ChannelAveragesFlow" multicastAddress="225.3.2.200"></ReceiverFlow>
        <ReceiverFlow Name="WVRFlow" multicastAddress="225.3.2.30"></ReceiverFlow>
    </ReceiverStream>
</TELCAL_DATAMANAGER_BDNT_1>
'''

root = etree.fromstring(xml)

# 原XPath，会返回所有继承了命名空间的元素
elements = root.xpath("//*[namespace-uri()!='' and not(local-name()='*')]")

for element in elements:
    print(element)

执行后返回所有带命名空间的元素（包括继承根命名空间的子元素），但我只需要自身显式声明xmlns="urn:schemas-cosylab-com:BulkDataNTReceiver:1.0"的TELCAL_DATAMANAGER_BDNT_1元素。

原XPath的问题在于，它筛选的是所有拥有非空命名空间的元素，而XML中子元素会自动继承父元素的默认命名空间，所以所有子元素都符合条件。要筛选自身显式声明了默认命名空间的元素，需要检查元素是否带有xmlns属性。

修改后的XPath表达式为：

//*[@xmlns]

或者更严谨地针对XML命名空间属性的特殊规则，使用：

//*[attribute::xmlns]

修改后的代码：

from lxml import etree

xml = '''
<TELCAL_DATAMANAGER_BDNT_1
    xmlns="urn:schemas-cosylab-com:BulkDataNTReceiver:1.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:schemas-cosylab-com:BulkDataNTReceiver:1.0 BulkDataNTReceiver.xsd">
    <ReceiverStream Name="InterferometricStream1" participantPerStream="true">
        <ReceiverFlow Name="FullSpectralFlow" cbReceiveAvgProcessTimeoutSec="0.0018" multicastAddress="225.3.2.100"></ReceiverFlow>
        <ReceiverFlow Name="ChannelAveragesFlow" multicastAddress="225.3.2.200"></ReceiverFlow>
        <ReceiverFlow Name="WVRFlow" multicastAddress="225.3.2.30"></ReceiverFlow>
    </ReceiverStream>
    <ReceiverStream Name="InterferometricStream2" participantPerStream="true">
        <ReceiverFlow Name="FullSpectralFlow" cbReceiveAvgProcessTimeoutSec="0.0018" multicastAddress="225.3.2.100"></ReceiverFlow>
        <ReceiverFlow Name="ChannelAveragesFlow" multicastAddress="225.3.2.200"></ReceiverFlow>
        <ReceiverFlow Name="WVRFlow" multicastAddress="225.3.2.30"></ReceiverFlow>
    </ReceiverStream>
    <ReceiverStream Name="InterferometricStream3" participantPerStream="true">
        <ReceiverFlow Name="FullSpectralFlow" cbReceiveAvgProcessTimeoutSec="0.0018" multicastAddress="225.3.2.100"></ReceiverFlow>
        <ReceiverFlow Name="ChannelAveragesFlow" multicastAddress="225.3.2.200"></ReceiverFlow>
        <ReceiverFlow Name="WVRFlow" multicastAddress="225.3.2.30"></ReceiverFlow>
    </ReceiverStream>
</TELCAL_DATAMANAGER_BDNT_1>
'''

root = etree.fromstring(xml)

# 修改后的XPath，仅返回自身显式声明默认命名空间的元素
elements = root.xpath("//*[@xmlns]")

for element in elements:
    print(element)

执行后输出：

<Element {urn:schemas-cosylab-com:BulkDataNTReceiver:1.0}TELCAL_DATAMANAGER_BDNT_1 at 0x7f8100613640>

补充说明 #

如果需要精准匹配特定的默认命名空间值，可以进一步过滤：

//*[@xmlns='urn:schemas-cosylab-com:BulkDataNTReceiver:1.0']

内容的提问来源于stack exchange，提问作者fjcaro