Insert a node for an element in XML with Python/ElementTree

Question

I need to traverse the XML tree to add sub element when the value is less than 5. For example, this XML can be modified into

<?xml version="1.0" encoding="UTF-8"?>
<A value="45">
    <B value="30">
        <C value="10"/>
        <C value ="20"/>
    </B>
    <B value="15">
        <C value = "5" />
        <C value = "10" />
    </B>
</A>

this XML.

<?xml version="1.0" encoding="UTF-8"?>
<A value="45">
    <B value="30">
        <C value="10"/>               
        <C value ="20"/>
    </B>
    <B value="15">
        <C value = "5"><D name="error"/></C>
        <C value = "10" />
    </B>
</A>

How can I do that with Python's ElementTree?

Can there be more than one <D> child? Have you considered the option of adding an "error" attribute to the element with a problem? — John Machin
– John Machin, Commented Jan 25, 2011 at 4:06

scoffey · Accepted Answer · 2011-01-25 01:38:32Z

13

You probably made a typo because in the example, an error element is appended as the child of an element whose value is 10, which is not less than 5. But I think this is the idea:

#!/usr/bin/env python

from xml.etree.ElementTree import fromstring, ElementTree, Element

def validate_node(elem):
    for child in elem.getchildren():
        validate_node(child)
        value = child.attrib.get('value', '')
        if not value.isdigit() or int(value) < 5:
            child.append(Element('D', {'name': 'error'}))

if __name__ == '__main__':
    import sys
    xml = sys.stdin.read() # read XML from standard input
    root = fromstring(xml) # parse into XML element tree
    validate_node(root)
    ElementTree(root).write(sys.stdout, encoding='utf-8')
            # write resulting XML to standard output

Given this input:

<?xml version="1.0" encoding="UTF-8"?>
<A value="45">
    <B value="30">
        <C value="1"/>
        <C value="20"/>
    </B>
    <B value="15">
        <C value="5" />
        <C value="10" />
        <C value="foo" />
    </B>
</A>

This is is the output:

<A value="45">
    <B value="30">
        <C value="1"><D name="error" /></C>
        <C value="20" />
    </B>
    <B value="15">
        <C value="5" />
        <C value="10" />
        <C value="foo"><D name="error" /></C>
    </B>
</A>

answered Jan 25, 2011 at 1:38

scoffey

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2 Comments

n611x007 Over a year ago

what I'm concerned about is that would an all-depth for loop iterate over the newly added child element here? eg. if the for is done with for node in list(tree.getroot()) and a node is added somewhere while iterating.

fedorqui Over a year ago

So the way to run this is cat file.xml | python script.py? I did and it works but I wonder if there is another way.

Mark Tolonen · Accepted Answer · 2011-01-25 05:49:19Z

2

ElementTree's iter (or getiterator for Python <2.7) willl recursively return all the nodes in a tree, then just test for your condition and create the SubElement:

from xml.etree import ElementTree as ET
tree = ET.parse(input)
for e in tree.getiterator():
    if int(e.get('value')) < 5:
        ET.SubElement(e,'D',dict(name='error'))

answered Jan 25, 2011 at 5:49

Mark Tolonen

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1 Comment

n611x007 Over a year ago

will the added element be yielded by the iterator? If so, how could I distinguish between new element and already existing ones?

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