Parsing XML with SAX and Python - The xml.sax Package

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SAX is a simple API for XML. The package xml.sax and its sub packages provide a Python implementation of the SAX interface.

The structure of a SAX application should include one or more input sources, parser and handler objects. The idea is as follows: a parser reads the bytes or characters from the input source and fires a sequence of events on the handler. In this document and in the Python documentation the term ‘reader’ is preferred over ‘parser’.

The SAX API defines four basic interfaces. Since Python does not support interfaces, these SAX interfaces are implemented in the xml.sax.handler module as the following Python classes:

1. ContentHandler: this implements the main SAX interface for handling document events. It is also the interface which we will use in the example of the next section
2. DTDHandler: class for handling DTD events
3. EntityResolver: class for resolving external entities
4. ErrorHandler: as the name suggests, this class is used for reporting all errors and warnings.

I would like to mention here the presence of the DefaultHandler class from the xml.sax.saxutils package that inherits from all four interfaces above. An application needs to implement only the interfaces it needs, as will be shown by the following example.

Now we have checked out the interfaces, it’s time to see the basic methods of the xml.sax package. These are:

make_parser() - This will create and return an SAX XMLReader object. Notice that the xml.sax readers are non-validating.

parse(filename, handler) - This will create a parser and parse the given document (which can be passed either as a file object or as a stream). The handler is one of the SAX interfaces we mentioned above.

A reader and a handler can be connected with the appropriate method (for example setContentHandler() for a ContentHandler object). Once this happens, the reader will notify of parsing events through the methods of the handler. In the following example, the methods startElement(), endElement() and characters() of the ContentHandler illustrate this procedure.

We will not go into error handling details in this document, but xml.sax provides enough exception classes for your programming needs. In the Python reference documentation you may find more details.

Enough with the theory, let’s move on to a hands-on example.

Next: Our SAX Parser >>
 

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