[lxml] a question about odd behaviour of 'break' in an lxml loop with iterchildren and itersiblings

Jan. 15, 2016

      I am puzzled by the following behaviour of lxml.  I want to group element
children with the same tag. Here is a schematic representation of my text:

<sp xml:id="sp-0645">
<speaker xml:id="spk-0645"/>
<ab xml:id="ftln-0645" n="2.1.1"/>
<ab xml:id="ftln-0646" n="2.1.2"/>
<ab xml:id="ftln-0647" n="2.1.3"/>
<ab xml:id="ftln-0648" n="2.1.4"/>
<stage/>
<ab xml:id="ftln-0703" n="2.1.59"/>
<ab xml:id="ftln-0704" n="2.1.60"/>
<stage/>
<ab xml:id="ftln-0705" n="2.1.61"/>
</sp>

I want to end up with something like :

<sp xml:id="sp-0645">
<speaker xml:id="spk-0645"/>
<p>
<lb xml:id="ftln-0645" n="2.1.1"/>
<lb xml:id="ftln-0646" n="2.1.2"/>
<lb xml:id="ftln-0647" n="2.1.3"/>
<lb xml:id="ftln-0648" n="2.1.4"/>
</p>
<stage/>
<p>
<lb xml:id="ftln-0703" n="2.1.59"/>
<lb xml:id="ftln-0704" n="2.1.60"/>
</p>
<stage/>
<p>
<lb xml:id="ftln-0705" n="2.1.61"/>
</p>
</sp>

I try to do this through a combination of iterchildren() and
itersiblings() and use the following script:

speech = 'speech.xml'
tree = etree.parse(speech)

for sp in tree.iter('sp'):
   for child in sp.iterchildren():
      if child.tag != 'ab':
          print('child', child.tag)

          for sibling in child.itersiblings():
              if sibling.tag == 'ab':
                 print('sibling'. sibling.tag)
              else:
                  break

The output of this script suggests that the two loops work properly:
child speaker
sibling ab
sibling ab
sibling ab
sibling ab
child stage
sibling ab
sibling ab
child stage
sibling ab

Now I complicate things by creating a paragraph element with each child
element.  The printout shows that each of the paragraphs exists and that
after the break statement the program returns to the top child loop:

child speaker
sibling ab
sibling ab
sibling ab
sibling ab
else <Element p at 0x1014cb548>
child stage
sibling ab
sibling ab
else <Element p at 0x1014cb508>
child stage
sibling ab
elif <Element p at 0x1014cb608>

But if I now add code to the script that fills the created paragraphs with
ab elements, the program does this correctly for the first iteration but
then it exits. Here is the code and the result:

speech = 'speech.xml'
tree = etree.parse(speech)

for sp in tree.iter('sp'):
    for child in sp.iterchildren():
        if child.tag != 'ab':
            paragraph = etree.Element('p')
            print('child', child.tag)

        for sibling in child.itersiblings():
            if sibling.tag == 'ab' and sibling.getnext()is not None:

                print('sibling', sibling.tag)
                paragraph.append(sibling)
            elif sibling.tag == 'ab' and sibling.getnext() is None:
                print('sibling', sibling.tag)
                paragraph.append(sibling)

                print('elif', etree.tostring(paragraph,
encoding='unicode', pretty_print=True))

            else:

                print('else',etree.tostring(paragraph, encoding='unicode',
pretty_print=True))

                Break

child speaker
sibling ab
sibling ab
sibling ab
sibling ab
else <p><ab xml:id="ftln-0645" n="2.1.1"/>

            <ab xml:id="ftln-0646" n="2.1.2"/>

            <ab xml:id="ftln-0647" n="2.1.3"/>

            <ab xml:id="ftln-0648" n="2.1.4"/>

            </p>

Why does the program execute only the first iteration of the loop and exit
completely after the 'break', when it doesn't do that in the structurally
identical but simpler versions?

[lxml] a question about odd behaviour of 'break' in an lxml loop with iterchildren and itersiblings

Martin Mueller