Download XML Processing with Python

XML Processing with Python
http://www.xml.com/pub/a/1999/12/xml99/python.html
by Sean McGrath
December 06, 1999
As part of our XML'99 coverage, we are pleased to bring you this taster from the
"Working with XML in Python" tutorial led by Sean McGrath.
Introduction
A century ago, when HTML and
CGI ruled the waves, Perl
dominated the Web programming
scene. As the transition to XML on the Web gathers pace, competition for the
hearts and minds of Web developers is heating up. One language attracting a lot
of attention at the moment is Python.
In this article we will take a high level look at Python. We will use the time
honored "Hello world" example program to illustrate the principle features of the
language. We will then examine the XML processing capabilities of Python.
Python is free
Python is free. You will find downloadable source code plus pre-compiled
executables on python.org. As you know, "free" is one of those words that is
often heavily loaded on the Internet. Fear not. Python is free with a capital "F".
You are free to do essentially anything you like with Python, including make
commercial use of it or derivatives created from it.
Python is interpreted
Python is an interpreted language. Programs can execute directly from the plain
text files that house them. Typically Python files have a .py extension. There is
no compilation phase as far as the programmer is concerned. Just edit and run!
Python is portable
Python is portable. It runs on basically every computing platform of note, from
mainframes to Palm Pilots and everything in between. Python uses a virtual
machine architecture, similar in concept to Java's virtual machine. The Python
interpreter "compiles" programs to virtual machine code on-the-fly. These
compiled files (typically having a .pyc extension) are also portable. That is to
say, if you wish to keep your source files hidden from your end-users you can
simply ship the compiled .pyc files.
Python is easy to understand
Python is very easy to understand. Here is a Python program that prints the
string "Hello world":
print "Hello world"
I think you will agree that programming a "Hello world" application cannot get
much simpler than that! To execute this program, you put it in a text file, say
Hello.py, and feed it to the Python interpreter like this:
python Hello.py
The output is, surprise, surprise:
Hello world
Note the complete lack of syntactic baggage in the Hello.py program. There are
no mandatory keywords or semi-colons required to get this simple job done. This
spartan, no-nonsense approach to syntax is one of the hallmarks of Python and
applies equally well to large Python programs.
Python is interactive
By invoking the Python interpreter (typically by typing python on a UNIX/Linux
system, or running the "IDLE" application on Windows), you will find yourself in
an environment where you can execute Python statements interactively. As an
example, here is the "Hello world" application again:
>>> print "Hello world"
This will output:
Hello world
Note that the ">>>" above is Python's command prompt. The interactive mode is
an excellent environment for playing around with Python. It is also indispensable
as a fully programmable calculator!
Python is WYSIWYG
Python is sometimes referred to as a WYSIWYG programming language. This is
because the indentation of Python code controls how the code is executed.
Python does not have begin/end keywords or braces for grouping code
statements. It simply does not need them. Take a look at the following Python
fragment:
if x > y:
print x
if y > z:
print y
print z
else:
print z
The indentation of the code is used to control how statements are grouped for
execution purposes. There can be no ambiguity as to which if clause is
associated with the else clause in the above code because both statements
have same level of indentation.
Functions in Python
We can turn the "Hello world" program into a Python function like this:
def Hello():
print "Hello world"
Note that statements within the body of a function are indented beneath the def
Hello() line which introduces the function. The parenthesis are a place holder
for function parameters. Here is a function that prints its parameters x and y as
well as the string "Hello world":
def Hello(x,y):
print "Hello world",x,y
Python modules
A Python program typically consists of a number of modules. Any Python source
file can serve as a module and be imported into another Python program. For
example, assuming the Hello function above is housed in the file Greeting.py
we can import the function into a Python program and call it as follows:
# Import the Hello function from the Greeting module
from Greeting import Hello
# Call the Hello function
Hello()
Programs as modules to larger programs
Python makes it easy to write programs that can be used both as stand-alone
programs and as modules to other programs.
Here is a modified version of Greeting.py which will print "Hello world" but can
also still be imported into other programs:
def Hello():
print "Hello world"
if __name__ == "__main__":
# Test Hello Function if running as
# main program
Hello()
Note the special __name__ variable above. This variable is automatically set to
"__main__" when a program is being executed directly. If it is being imported
into another program, __name__ is set to the name of the module, which in this
case would be "Greeting".
Python is object-oriented
Python is a very object-oriented language. Here is an extended version of the
"Hello world" program, called Message.py, that can print any message via
MessageHolder objects:
#Create a class called MessageHolder
class MessageHolder:
# Constructor - called automatically
# when an object of this class is created
def __init__(self,msg):
self.msg = msg
# Function to return the stored message string
def getMsg(self):
return self.msg
Note how indentation is used to structure the source code. the getMsg function is
associated with objects of the MessageHolder class because it is indented
beneath the class MessageHolder. Functions associated with objects are more
generally known as methods.
Suppose now that I need a variation on the MessageHolder class in which all
messages are returned in uppercase. I can do that by subclassing
MessageHolder, specifying the class I wish to inherit from in parentheses after
the class name:
# Import existing MessageHolder class from Message.py
from Message import MessageHolder
# Create a sub-class of MessageHolder called MessageUpper
class MessageUpper(MessageHolder):
# Constructor
def __init__(self,msg):
# Call constructor of superclass
Message.__init__(msg)
# Over-ride getMsg with new
# functionality
def getMsg(self):
return string.upper(self.msg)
Python is extensible
The Python language consists of a small core and a large collection of modules.
Some of these modules are written in Python and some are written in C. As a
user of Python modules, you cannot tell the difference. For example:
import xmlproc
import pyexpat
The first statement imports Lars Marius Garshol's implementation of an XML
parser that is written purely in Python. The second statement imports the Python
wrapping of James Clark's expat XML parser which is written in C.
Python programs using these modules cannot tell what language they have been
implemented in. As you would expect, programs based on expat are typically
faster owing to the speed advantages of a pure C implementation of an XML
parser.
It is remarkably easy to write a Python module in C. This facility is very useful for
speed-critical parts of large Python systems. It is also easy to "wrap" existing C
libraries as Python modules, as has been done with expat. Many technologies
exposing a C API have been wrapped as Python modules, for example Oracle,
the Win32 API, and the wxWindows GUI toolkit, to name a few.
XML programming support
The core Python distribution (currently at version 1.5.2) has a simple nonvalidating XML parser module called xmllib. The vast bulk of Python's XML
support is in the form of an add-on module under active development by the SIG
for XML Processing in Python (known as XML-SIG). To illustrate Python's XML
support, we will switch to an XML 1.0 version of the "Hello world" program
processing the following file:
<?xml version = "1.0"?>
<Greeting>
Hello world
</Greeting>
SAX
SAX is a simple API for XML, spearheaded by David Megginson and developed
as a collaborative effort on the XML-dev mail list. The Python implementation
was developed by Lars Marius Garshol.
A Python SAX application to count the words in Greeting.xml looks like this:
from xml.sax import saxexts, saxlib, saxutils
import string
# Create a class to handle document events
class docHandler(saxlib.DocumentHandler):
# Start of document handler
def startDocument(self):
# Initialize storage for character data
self.Storage = ""
# end of document handler
def endDocument(self):
# Print approximate number of words
# by counting the number of elements in
# the list of words returned by the
# string.split function
print len(string.split(self.Storage))
def characters(self,str,start,end):
# Accumulate character data
self.Storage = self.Storage + str[start:end]
# Create a parser
parser = saxexts.make_parser()
# Provide the parser with a document handler
parser.setDocumentHandler(docHandler())
# Parse the Greeting.xml file
parser.parseFile(open("Greeting.xml"))
DOM
The DOM is a W3C initiative to standardize an API to XML (and HTML)
documents. Python has two DOM implementations. The one in the XML-SIG
modules is the work of Andrew Kuchling and Stéfane Fermigier. The other is
called 4DOM and is the work of Fourthought, who have also created XSLT and
XPath implementations in Python.
Here is a sample DOM application to count the words in Greeting.xml:
from xml.dom import utils,core
import string
# Read an XML document into a DOM object
reader = utils.FileReader('Greeting.xml')
# Retrieve top level DOM document object
doc = reader.document
Storage = ""
# Walk over the nodes
for n in doc.documentElement.childNodes:
if n.nodeType == core.TEXT_NODE:
# Accumulate contents of text nodes
Storage = Storage + n.nodeValue
print len(string.split(Storage))
Native Python APIs
As well as industry standard APIs, there is a native Python XML processing
library known as Pyxie.
Pyxie is an open source XML processing library for Python which will be made
publicly available in January 2000. Pyxie tries to make the best of Python's
features to simplify XML processing.
Here is the word counting application developed using Pyxie:
from pyxie import *
# Load XML into tree structure
t = File2xTree("Greeting.xml")
Storage = ""
# Iterate over list of data nodes
for n in Data(t):
Storage = Storage + t.Data
print len(string.split(Storage))
In conclusion
We have looked at some of the main features of Python in a high level way. Also,
we have glimpsed at some of the XML processing facilities available. For further
information on programming with Python, I suggest you start with
http://www.python.org.