How can one replace an element with text in lxml?

Question

It's easy to completely remove a given element from an XML document with lxml's implementation of the ElementTree API, but I can't see an easy way of consistently replacing an element with some text. For example, given the following input:

input = '''<everything>
<m>Some text before <r/></m>
<m><r/> and some text after.</m>
<m><r/></m>
<m>Text before <r/> and after</m>
<m><b/> Text after a sibling <r/> Text before a sibling<b/></m>
</everything>
'''

... you could easily remove every <r> element with:

from lxml import etree
f = etree.fromstring(data)
for r in f.xpath('//r'):
    r.getparent().remove(r)
print etree.tostring(f, pretty_print=True)

However, how would you go about replacing each element with text, to get the output:

<everything>
<m>Some text before DELETED</m>
<m>DELETED and some text after.</m>
<m>DELETED</m>
<m>Text before DELETED and after</m>
<m><b/>Text after a sibling DELETED Text before a sibling<b/></m>
</everything>

It seems to me that because the ElementTree API deals with text via the .text and .tail attributes of each element rather than nodes in the tree, this means you have to deal with a lot of different cases depending on whether the element has sibling elements or not, whether the existing element had a .tail attribute, and so on. Have I missed some easy way of doing this?

Answer 1

Using ET.XSLT:

import io
import lxml.etree as ET

data = '''<everything>
<m>Some text before <r/></m>
<m><r/> and some text after.</m>
<m><r/></m>
<m>Text before <r/> and after</m>
<m><b/> Text after a sibling <r/> Text before a sibling<b/></m>
</everything>
'''

f=ET.fromstring(data)
xslt='''\
    <xsl:stylesheet version="1.0"
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform">    

    <!-- Replace r nodes with DELETED
         http://www.w3schools.com/xsl/el_template.asp -->
    <xsl:template match="r">DELETED</xsl:template>

    <!-- How to copy XML without changes
         http://mrhaki.blogspot.com/2008/07/copy-xml-as-is-with-xslt.html -->    
    <xsl:template match="*">
        <xsl:copy>
            <xsl:apply-templates select="@*|node()"/>
        </xsl:copy>
    </xsl:template>
    <xsl:template match="@*|text()|comment()|processing-instruction">
        <xsl:copy-of select="."/>
    </xsl:template>
    </xsl:stylesheet>
'''

xslt_doc=ET.parse(io.BytesIO(xslt))
transform=ET.XSLT(xslt_doc)
f=transform(f)

print(ET.tostring(f))

yields

<everything>
<m>Some text before DELETED</m>
<m>DELETED and some text after.</m>
<m>DELETED</m>
<m>Text before DELETED and after</m>
<m><b/> Text after a sibling DELETED Text before a sibling<b/></m>
</everything>

Answer 2

I think that unutbu's XSLT solution is probably the correct way to achieve your goal.

However, here's a somewhat hacky way to achieve it, by modifying the tails of <r/> tags and then using etree.strip_elements.

from lxml import etree

data = '''<everything>
<m>Some text before <r/></m>
<m><r/> and some text after.</m>
<m><r/></m>
<m>Text before <r/> and after</m>
<m><b/> Text after a sibling <r/> Text before a sibling<b/></m>
</everything>
'''

f = etree.fromstring(data)
for r in f.xpath('//r'):
  r.tail = 'DELETED' + r.tail if r.tail else 'DELETED'

etree.strip_elements(f,'r',with_tail=False)

print etree.tostring(f,pretty_print=True)

Gives you:

<everything>
<m>Some text before DELETED</m>
<m>DELETED and some text after.</m>
<m>DELETED</m>
<m>Text before DELETED and after</m>
<m><b/> Text after a sibling DELETED Text before a sibling<b/></m>
</everything>

Answer 3

Using strip_elements has the disadvantage that you cannot make it keep some of the <r> elements while replacing others. It also requires the existence of an ElementTree instance (which may be not the case). And last, you cannot use it to replace XML comments or processing instructions. The following should do your job:

for r in f.xpath('//r'):
    text = 'DELETED' + r.tail 
    parent = r.getparent()
    if parent is not None:
        previous = r.getprevious()
        if previous is not None:
            previous.tail = (previous.tail or '') + text
        else:
            parent.text = (parent.text or '') + text
        parent.remove(r)