9.5. Searching for elements

Traversing XML documents by stepping through each node can be tedious. If you're looking for something in particular, buried deep within your XML document, there is a shortcut you can use to find it quickly: getElementsByTagName.

For this section, you'll be using the binary.xml grammar file, which looks like this:

Example 9.20. binary.xml

<?xml version="1.0"?>
<!DOCTYPE grammar PUBLIC "-//book.diveintopython.org//DTD Kant Generator Pro v1.0//EN" "kgp.dtd">
<grammar>
<ref id="bit">
  <p>0</p>
  <p>1</p>
</ref>
<ref id="byte">
  <p><xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/>\
<xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/></p>
</ref>
</grammar>

It has two refs, 'bit' and 'byte'. A bit is either a '0' or '1', and a byte is 8 bits.

Example 9.21. Introducing getElementsByTagName

>>> from xml.dom import minidom
>>> xmldoc = minidom.parse('binary.xml')
>>> reflist = xmldoc.getElementsByTagName('ref') 1
>>> reflist
[<DOM Element: ref at 136138108>, <DOM Element: ref at 136144292>]
>>> print reflist[0].toxml()
<ref id="bit">
  <p>0</p>
  <p>1</p>
</ref>
>>> print reflist[1].toxml()
<ref id="byte">
  <p><xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/>\
<xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/></p>
</ref>
1 getElementsByTagName takes one argument, the name of the element you wish to find. It returns a list of Element objects, corresponding to the XML elements that have that name. In this case, you find two ref elements.

Example 9.22. Every element is searchable

>>> firstref = reflist[0]                      1
>>> print firstref.toxml()
<ref id="bit">
  <p>0</p>
  <p>1</p>
</ref>
>>> plist = firstref.getElementsByTagName("p") 2
>>> plist
[<DOM Element: p at 136140116>, <DOM Element: p at 136142172>]
>>> print plist[0].toxml()                     3
<p>0</p>
>>> print plist[1].toxml()
<p>1</p>
1 Continuing from the previous example, the first object in your reflist is the 'bit' ref element.
2 You can use the same getElementsByTagName method on this Element to find all the <p> elements within the 'bit' ref element.
3 Just as before, the getElementsByTagName method returns a list of all the elements it found. In this case, you have two, one for each bit.

Example 9.23. Searching is actually recursive

>>> plist = xmldoc.getElementsByTagName("p") 1
>>> plist
[<DOM Element: p at 136140116>, <DOM Element: p at 136142172>, <DOM Element: p at 136146124>]
>>> plist[0].toxml()                         2
'<p>0</p>'
>>> plist[1].toxml()
'<p>1</p>'
>>> plist[2].toxml()                         3
'<p><xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/>\
<xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/></p>'
1 Note carefully the difference between this and the previous example. Previously, you were searching for p elements within firstref, but here you are searching for p elements within xmldoc, the root-level object that represents the entire XML document. This does find the p elements nested within the ref elements within the root grammar element.
2 The first two p elements are within the first ref (the 'bit' ref).
3 The last p element is the one within the second ref (the 'byte' ref).