Python ElementTree: find element by its child's text using XPath

Question

I'm trying to locate an element that has certain text value in one of its child. For example,

<peers>
    <peer>
        <offset>1</offset>
        <tag>TRUE</tag>
    </peer>
    <peer>
        <offset>2</offset>
        <tag>FALSE</tag>
    </peer>
</peers>

from this XML document I would like to directly locate tag in a peer element whose offset value is 1.

So for that purpose I have a XPath expression as follows:

./peers/peer[offset='1']/tag

however using such expression in ElementTree's Element.find() method fails and gives None rather than the "tag" element of my interest:

from xml.etree.ElementTree import fromstring

doc = fromstring("<peers><peer><offset>1</offset><tag>TRUE</tag></peer><peer><offset>2</offset><tag>FALSE</tag></peer></peers>")

tag = doc.find("./peers/peer[offset='1']/tag")

print tag


=> None

I'm being inclined to believe it's either my above XPath expression is wrong, or due to ElementTree's supporting only a subset of XPath according to its documentation. Looking for help. Thank you.

Answer 1

Using lxml.etree directly (the same should apply to ElementTree), you can achieve the result like this:

doc = lxml.etree.fromstring(...)
tag_elements = doc.xpath("/peers/peer/offset[text()='1']/../tag")

tag_elements will be the list of <tag> elements belonging to <peer> elements containing an <offset> element containing 1.

Given input (I've added a <peer> clause to emphasize tag_elements being a list):

<peers>
    <peer>
        <offset>1</offset>
        <tag>TRUE</tag>
    </peer>
    <peer>
        <offset>1</offset>
        <tag>OTHER</tag>
    </peer>
    <peer>
        <offset>2</offset>
        <tag>FALSE</tag>
    </peer>
</peers>

tag_elements will contain two elements:

for tag in tag_elements:
    print tag.text
-> TRUE
-> OTHER

UPDATE:

doc.xpath("/peers/peer[offset=1]/tag") also works fine.

But doc.xpath("./peers/peer[offset=1]/tag") does not.