Question

I’m reading instrument data from a specialty server that delivers the info in xml format. The code I’ve written is:
from lxml import etree as ET

xmlDoc = ET.parse('http://192.168.1.198/Bench_read.xml')
print ET.tostring(xmlDoc, pretty_print=True)

dmtCount = xmlDoc.xpath('//dmt')
print(len(dmtCount))

dmtVal = []

for i in range(1, len(dmtCount)):
    dmtVal[i:0] = xmlDoc.xpath('./address/text()')
    dmtVal[i:1] = xmlDoc.xpath('./status/text()')
    dmtVal[i:2] = xmlDoc.xpath('./flow/text()')
    dmtVal[i:3] = xmlDoc.xpath('./dp/text()')
    dmtVal[i:4] = xmlDoc.xpath('./inPressure/text()')
    dmtVal[i:5] = xmlDoc.xpath('./actVal/text()')
    dmtVal[i:6] = xmlDoc.xpath('./temp/text()')
    dmtVal[i:7] = xmlDoc.xpath('./valveOnPercent/text()')

print dmtVal

And the results I get are:

$python XMLparse2.py
<response>

<heartbeat>0x24</heartbeat>

<dmt node="1">

    <address>0x21</address>
    <status>0x01</status>
    <flow>0.000000</flow>
    <dp>0.000000</dp>
    <inPressure>0.000000</inPressure>
    <actVal>0.000000</actVal>
    <temp>0x00</temp>
    <valveOnPercent>0x00</valveOnPercent>

</dmt>

<dmt node="2">

    <address>0x32</address>
    <status>0x01</status>
    <flow>0.000000</flow>
    <dp>0.000000</dp>
    <inPressure>0.000000</inPressure>
    <actVal>0.000000</actVal>
    <temp>0x00</temp>
    <valveOnPercent>0x00</valveOnPercent>

</dmt>

</response>

...Starting to parse XML nodes
2
[]
...Done

Sooo, nothing is coming out. I’ve tried using /value in place of the /text() in the xpath call, but the results are unchanged. Is my problem:

1) An incorrect xpath command in the for loop? or

2) A problem in the way I’ve structured list variable dmtVal ? or

3) Something else I’m missing completely?

I’d welcome any suggestions! Thanks in advance…

Answer 1

Counting <dmt/> tags and then iterating over them by index is both inefficient and un-Pythonic. Apart from that you are using wrong syntax (slice instead of index) for indexing arrays. In fact you don’t need to index the val at all, to do it Pythonic way use list comprehensions.

Here’s a slightly modified version of what stranac suggested:

from lxml import etree as ET

xmlDoc = ET.parse('http://192.168.1.198/Bench_read.xml')
print ET.tostring(xmlDoc, pretty_print=True)

response = xmlDoc.getroot()

tags = (
    'address',
    'status',
    'flow',
    'dp',
    'inPressure',
    'actVal',
    'temp',
    'valveOnPercent',
)

dmtVal = []

for dmt in response.iter('dmt'):
    val = [dmt.xpath('./%s/text()' % tag) for tag in tags]
    dmtVal.append(val)