如何使用lxml.etree获取第n-1个节点的value值

Got it, let's walk through exactly how to pull the value you need from your XML structure using lxml.etree. Here's a step-by-step breakdown with practical code examples:

Step 1: Import lxml.etree and Parse Your XML #

First, make sure you have lxml installed (run pip install lxml if you haven't already). Then parse your XML—whether it's stored as a string or in a file:

Option A: Parse from a string #

from lxml import etree

# Your XML content as a raw string
xml_str = """<root> 
<test> 
<criteria nom="DR"> 
<abbr>DR</abbr> 
<value>0.123456</value> 
</criteria> 
<criteria nom="MOTA"> 
<abbr>MOTA</abbr> 
<value>0.132465</value> 
</criteria> 
<criteria nom="PFR"> 
<abbr>PFR</abbr> 
<value>0.914375</value> 
</criteria> 
</test> 
<test> 
<criteria nom="DR"> 
<abbr>DR</abbr> 
<value>0.655425</value> 
</criteria>
</test></root>"""

root = etree.fromstring(xml_str)

Option B: Parse from a file #

from lxml import etree

# Load and parse directly from an XML file
tree = etree.parse("your_xml_file.xml")
root = tree.getroot()

Step 2: Target the (n-1)th <test> Node #

Python uses 0-indexing, so the (n-1)th position in the list of <test> nodes maps to the nth logical test node. For example, if you want the 2nd test node, you'll use index 1 (since n=2 → n-1=1):

n = 2  # Replace with your desired n value
try:
    target_test = root.xpath("./test")[n-1]
except IndexError:
    print(f"Error: There is no {n}th <test> node in your XML.")
    exit()

Step 3: Extract the Value(s) #

Now you can pull the value(s) from the targeted <test> node. Here are two common use cases:

Option 1: Get all <value> texts in the target test node #

If you want every value under the (n-1)th test:

all_values = [crit.find("value").text for crit in target_test.findall("criteria")]
print(f"All values in the {n}th test node: {all_values}")
# Output for n=2: All values in the 2nd test node: ['0.655425']

Option 2: Get a specific value by <criteria>'s nom attribute #

If you only need the value for a specific criteria (like "DR" or "MOTA"):

# Get value for criteria with nom="DR"
dr_value = target_test.xpath('./criteria[@nom="DR"]/value/text()')[0]
print(f"DR value in {n}th test node: {dr_value}")
# Output for n=2: DR value in 2nd test node: 0.655425

Pro Tip: Add Error Handling #

To avoid crashes if the desired criteria doesn't exist in the test node, wrap the lookup in a try-except block:

try:
    mota_value = target_test.xpath('./criteria[@nom="MOTA"]/value/text()')[0]
    print(f"MOTA value: {mota_value}")
except IndexError:
    print(f"No MOTA criteria found in the {n}th test node.")

内容的提问来源于stack exchange，提问作者Greg Symsym