Beginning Python - From Novice to Professional

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CHAPTER 22 ■ PROJECT 3: XML FOR ALL OCCASIONS 425 WHAT ABOUT DOM? There are two common ways of dealing with XML in Python (and other programming languages, for that matter): SAX and DOM (the Document Object Model). A SAX parser reads through the XML file and tells you what it sees (text, tags, attributes), storing only small parts of the document at a time; this makes SAX simple, fast, and memory-efficient, which is why I have chosen to use it in this chapter. DOM takes another approach: It constructs a data structure (the document tree), which represents the entire document. This is slower and requires more memory, but can be useful if you want to manipulate the structure of your document, for example. For information about using DOM in Python, check out the Python Library Reference (http:// www.python.org/doc/lib/module-xml.dom.html). In addition to the standard DOM handling, the standard library contains two other modules, xml.dom.minidom (a simplified DOM) and xml.dom.pulldom (a cross between SAX and DOM, which reduces memory requirements). A very fast and simple XML parser (which doesn’t really use DOM, but which creates a complete document tree from your XML document) is PyRXP (http://www.reportlab.org/pyrxp.html). And then there is ElementTree (available from http://effbot.org/zone), which is flexible and easy to use. Creating a Simple Content Handler There are several event types available when parsing with SAX, but let’s restrict ourselves to three: the beginning of an element (the occurrence of an opening tag), the end of an element (the occurrence of a closing tag), and plain text (characters). To parse the XML file, let’s use the parse function from the xml.sax module. This function takes care of reading the file and generating the events—but as it generates these events, it needs some event handlers to call. These event handlers will be implemented as methods of a content handler object. You’ll subclass the ContentHandler class from xml.sax.handler because it implements all the necessary event handlers (as dummy operations that have no effect), and you can override only the ones you need. Let’s begin with a minimal XML parser (assuming that your XML file is called website.xml): from xml.sax.handler import ContentHandler from xml.sax import parse class TestHandler(ContentHandler): pass parse('website.xml', TestHandler()) If you execute this program, seemingly nothing happens, but you shouldn’t get any error messages either. Behind the scenes, the XML file is parsed, and the default event handlers are called—but because they don’t do anything, you won’t see any output. Let’s try a simple extension. Add the following method to the TestHandler class: def startElement(self, name, attrs): print name, attrs.keys()
426 CHAPTER 22 ■ PROJECT 3: XML FOR ALL OCCASIONS This overrides the default startElement event handler. The parameters are the relevant tag name and its attributes (kept in a dictionary-like object). If you run the program again (using website.xml from Listing 22-1), you see the following output: website [] page [u'name', u'title'] h1 [] p [] ul [] li [] a [u'href'] li [] a [u'href'] li [] a [u'href'] directory [u'name'] page [u'name', u'title'] h1 [] p [] page [u'name', u'title'] h1 [] p [] page [u'name', u'title'] h1 [] p [] How this works should be pretty clear. In addition to startElement, you’ll use endElement (which takes only a tag name as its argument) and characters (which takes a string as its argument). The following is an example that uses all these three methods to build a list of the headlines (the h1 elements) of the Web site file: from xml.sax.handler import ContentHandler from xml.sax import parse class HeadlineHandler(ContentHandler): in_headline = False def __init__(self, headlines): ContentHandler.__init__(self) self.headlines = headlines self.data = []
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Beginning Python From Novice to Pro
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For Ranveig
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viii ■CONTENTS Strings . . . . .
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256 CHAPTER 11 ■ FILES AND STUFF
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270 CHAPTER 12 ■ GRAPHICAL USER I
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298 CHAPTER 14 ■ NETWORK PROGRAMM
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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