Why should I use XPathContext with Perl's XML::LibXML?

Question

This script works with and without XPathContext. Why should I use it with XPathContext?

#!/usr/bin/env perl
use warnings; use strict;
use XML::LibXML;
use 5.012;

my $parser = XML::LibXML->new;

my $doc = $parser->parse_string(<<EOT);
<?xml version="1.0"?>
<xml>
    Text im Dokument
    <element id="myID" name="myname" style="old" />
    <object objid="001" objname="Object1" />
    <element id="002" name="myname" />
</xml>
EOT 
#/

# without XPathContext
my $nodes = $doc->findnodes( '/xml/element[@id=002]' );

# with XPathContext
#my $root = $doc->documentElement;
#my $xc = XML::LibXML::XPathContext->new( $root );
#my $nodes = $xc->findnodes( '/xml/element[@id=002]' );


for my $node ( $nodes->get_nodelist ) {
    say "Node: ", $node->nodeName;
    print "Attribute: ";
    print $_->getName, '=', $_->getValue, ' ' for    $node->attributes;
    say "";
}

Answer 1

The primary reason for using XPathContext elements is namespaces. Your document has no namespaces so XPathContexts don't add anything to your query. Now, imagine that you actually had the following xml

my $doc = $parser->parse_string(<<EOT);
<?xml version="1.0"?>
<xml xmlns="http://my.company.com/ns/nsone" 
    xmlns:ns2="http://your.company.com/nstwo"&gt;
    Text im Dokument
    <ns2:element id="myID" name="myname" style="old" />
    <object objid="001" objname="Object1" />
    <element id="002" name="myname" />
</xml>
EOT 

You would need to define an XPathContext in order to have namespaces defined so that you could make namespace aware XPath queries:

my $root = $doc->documentElement;
my $xc = XML::LibXML::XPathContext->new( $root );
$xc->registerNs("ns2", "http://your.company.com/nstwo");
$xc->registerNs("ns1", "http://my.company.com/nsone");
my $nodes = $xc->findnodes( '/ns1:xml/ns2:element[@id="myID"]' );

Otherwise, you have no simple way to use namespace aware XPath statements.