Python Cookbook

Counting Tags in a Document

Credit: Paul Prescod

Problem

You want to get a sense of how often particular elements occur in an XML document, and the relevant counts must be extracted rapidly.

Solution

You can subclass SAX’s ContentHandler to make your own specialized classes for any kind of task, including the collection of such statistics:

from xml.sax.handler import ContentHandler
import xml.sax

class countHandler(ContentHandler):
    def _ _init_ _(self):
        self.tags={}

    def startElement(self, name, attr):
        if not self.tags.has_key(name):
            self.tags[name] = 0
        self.tags[name] += 1

parser = xml.sax.make_parser(  )
handler = countHandler(  )
parser.setContentHandler(handler)
parser.parse("test.xml")

tags = handler.tags.keys(  )
tags.sort(  )
for tag in tags:
 print tag, handler.tags[tag]

Discussion

When I start with a new XML content set, I like to get a sense of which elements are in it and how often they occur. I use variants of this recipe. I can also collect attributes just as easily, as you can see. If you add a stack, you can keep track of which elements occur within other elements (for this, of course, you also have to override the endElement method so you can pop the stack).

This recipe also works well as a simple example of a SAX application, usable as the basis for any SAX application. Alternatives to SAX include pulldom and minidom. These would be overkill for this simple job, though. For any simple processing, this is generally the case, particularly if the document you are processing is ...