python and xml

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Python and XML

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About me

• Paul Prescod, (paul@activestate.com)

• ActiveState Senior Developer

• Co-Author, XML Handbook

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What is Python?

• Python is an easy to learn, powerful

programming language

– Efficient high-level data structures

– Simple approach to object-oriented programming

– Elegant syntax and dynamic typing

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Brief History of Python

• CWI, early 90s

• Dynamic Object Oriented High Level Language

• More than a text processing language

• More than a scripting language

• Scalable and object oriented from the beginning

• Dynamically type checked

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Python's business case

• Python can displace many other

languages in the organization

• The Python interpreter is free

• Python is legally unencumbered

• Professional programmers find Python more flexible than most languages

• Amateur programmers are (often) more comfortable than with Perl or Java

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Usability features

• Exceptionally clear syntax

• Provides an obvious way to do most things

• Small set of features combine in

powerful ways

• Only innovative where innovation is

really necessary

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More Usability features

• Huge amount of free code and libraries

• Interactive

• Designed to talk to the world

• Runs with Unix, Mac and Windows

• Integrates with JVM (Jython) and NET Framework (Python.NET)

• Talks MS COM, XPCOM,

CORBA,SOAP, XML-RPC, …

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Scalability features

• Simple but powerful module system

• Simple but powerful class system

• Structured, standardized exceptions

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Extendable

• New data types in Python or C

• Modules in Python or C

• Functions in Python or C

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Python isn't picky!

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Compared to Java

• Java is more difficult for amateur

programmers

• Static type checking can be

inconvenient in text processing

• Puritanical OO can be inconvenient

• Bottom line: Java can make simple

projects harder

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Why not Java: political

• "100% pure Java" gets in the way

• The Java environment punishes

interoperability (e.g getenv is

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Jython (nee JPython)

• Compiles Python classes to Java

classes

• Embedded interpreter allows interactive coding

• Access to all Java classes

• For better or worse: maintains Java's

security/platform-independence bubble

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Jython can use Java tools

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• Raw text searching is not as fast as Perl.

• Dynamic type checking requires more care in testing.

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Python “Hello world"

print "Hello, World“

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Python interpreter

• Just type:

C:\> python

Python 1.5.2 (#0, Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)] on win32

>>> print "Hello, World"

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• py files get a pyc in the same directory

• When the py is updated, the pyc is

updated

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Interpreters

• DOS/Win32 (last slide)

• Unix (use ^D to exit)

• Graphical: “IDLE”, “PythonWin”

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Numeric types

• int: 32 bit, e.g "x=5"

• long: arbitrary sized, e.g "x=2L**128"

• float: accuracy depends on platform, e.g "x=3.14"

• complex: real+imag., "x=5.3+3.2j"

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Sequence types:

• Strings: "abcd"

• Tuples: (1,2,"b")

• Lists: [1,"a",3]

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Sequence types: string

myStr = "abc" # assignment

myStr = myStr + "def" # = "abcdef"

otherstr = myStr[ 1 : 4 ] # = "bcd"

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Sequence types: lists

myList = ["a", 5 , 3.25 , 2L , 4 + 3j ]

anotherList = ["a",myList, ["3","2"]] anotherList2 = myList + myList

# = ["a",5, ,"a",5, ]

yetAnotherList = myList[ 1 : 3 ]

# = [5,3.25]

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Iterating over sequences

strlist = ["abc", "def", "ghi"]

for item in strlist:

for char in item:

print char

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Getting the length

• The len() function gets a sequence's length

>>> len( "abc" )

3

>>> len( ["abc","def"] )

2

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Traceback (innermost last):

File "<stdin>", line 1, in ?

TypeError: object doesn't support item assignment

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Dictionaries

• Serve as a lookup table

• Maps "keys" to "values"

• Keys can be of any immutable type

• Assignment adds or changes members

• keys() method returns keys

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'a' : 'alpha' }

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>>> dict.clear()

>>> print dict

{}

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File Objects

• Represent opened files:

myFile = open( "catalog.txt", "r" )

data = myFile.read()

myFile = open( "catalog2.txt", "w" )

data = data+ "more data"

myFile.write( data )

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Function definitions

• Encapsulate bits of code

• Can take a fixed or variable number of arguments

• Arguments can have default values

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Functions are objects

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Flow Control Statements

• if/then/else

• while

• for

• try

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Exception handling

• Python exception handling like Java/C++

• Errors are reported in tracebacks

• Exceptions propagate up

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Exception traceback

Traceback (innermost last):

File "test.py", line 10, in ?

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Classes

• Classes combine code and data.

• They represent real world objects.

• We create "instance objects" from classes.

• Closest languages in terms of object model are SmallTalk or Ruby.

• Much more flexible than Java or C++

• More central to the language than

Perl/Tcl/PHP.

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Inheritance

• Classes can specify a base class.

• The new class "inherits" methods and data.

• The new class can

– "override" methods.

– add data and methods.

• Multiple Inheritance is okay

• All methods are virtual.

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Modules and Packages

• A module is a set of code in a single file

• A package is a collection of related

modules

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XML and Python

• Accessing XML with Python

• Parsing XML with Python

– Non validating Parsers

– Validating Parsers

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Parsers for Jython

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Manipulating XML

• Flat file processing with RE's (briefly!)

• PySAX - Simple API for XML

• PyDOM - W3C Document Object Model

• …

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Flat File Processing

• XML documents are text

• Ordinary textual tools continue to work

• E.G Search for emph elements:

for i in re.search(

r"<emph>(.*)</emph>" , input ):

print i

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Flat File Recipe

• Unless your needs are very simple, let

me help you!

• I’ve already converted the ultimate XML parsing regular expression to Python:

http://aspn.activestate.com/ASPN/Python/Cookbook/Recipe/65125

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Events

• Think of an XML document as a series

of events

• "Start tag", "End tag", “Characters", etc

• We can handle hierarchy by tracking start/end tags

• We can deal with the document a little

at a time

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PySAX

• "Simple API for XML"

• Common API for parsers

• Based on Java API

• Parser implements certain interfaces

• Application implements callback

interfaces

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SAX Model

• The application hands the parser an

event handler object

• The parser sends events to the handler

• The handler can

– store them somehow,

– build something,

– re-route them to other parts of the

app

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print handler.tags

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ErrorHandling

• In addition to content handler,

• we should assign an error handler.

class MyErrorHandler:

def warning(self, exception):

print "Whoa, nelly!"

print exception

def error(self, exception):

print "Whoa, nelly!"

raise exception

def fatalError(self, exception):

print "Whoa, nelly!" raise exception

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Character handling

# print out characters in document

from xml.sax.handler import ContentHandler

import xml.sax, sys

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Document Object Model

• Document Object Model

• The DOM is a W3C standard

• Extended version of "Dynamic HTML"

• Defined in CORBA IDL

• Implemented in various languages

• Implemented in IE5.0 and eventually Netscape

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The DOM

• The DOM is a tree-based API

• This implies a certain amount of

overhead

• But also a lot of convenience and

flexibility

• XPath implementation essentially

requires tree-based APIs

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DOM Nodes

• Elements, attributes, comments, etc

called "nodes"

• Classes represent node types

• All node types subclass the "node" base class

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DOM node types

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More DOM node types

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Navigation properties

• parentNode - Parent of this node

• firstChild - First child of this node

• lastChild - Last child of this node

• previousSibling - Node immediately preceding this node

• nextSibling - Node immediately following this node

• childNodes - List containing all the children of this node

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<title> SIG for XML Processing in

Python </title>

</bookmark>

</folder>

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First "title" node

Properties:

• parentNode: folder element

• firstChild: Text node 'XML bookmarks'

• lastChild: Text node 'XML bookmarks'

• previousSibling: codeNone

• nextSibling: bookmark element

• childNodes: A 1-element list: [ Text node

'XML bookmarks' ]

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Modifying a DOM

appendChild(newChild)

insertBefore(newChild, refChild) replaceChild(newChild, oldChild)

removeChild(oldChild)

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The Document Node

• One Document node per document

• The base of the entire tree

• documentElement attribute contains a single Element node

• childNodes may have additional

children, such as ProcessingInstruction nodes

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PyDOM

• A richer, more robust DOM than

minidom

• More classes, support for DOM 2+

• Integration with XPath and XSLT

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PyXML Parsers

• Xml.parsers.xmlproc

• Qp_xml

• Xml.sax.drivers

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– …

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Python SOAP Example

• SOAP.py:

import SOAP

server =

SOAP.SOAPProxy( "http://local host:8000/" )

print server.echo( "Hello

world" )

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XML and Zope

• Zope is an Open Source application server

that publishes objects on the Internet.

• ParsedXML: Breaks up an XML document

into bits.

• XML-RPC: You can plumb the depths of Zope with XML-RPC.

• Zcatalog: Index based on element-type

names, attribute names, etc.

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ParsedXML

• A free Zope “product” (extension)

• Every element is a first-class Zope

object

• You can add “behavior” to XML

documents

• RSS Channel Product

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Redfoot

• Redfoot is a framework for distributed based applications, written in Python.

RDF-– an RDF database

– a query API for RDF

– an RDF parser and serializer

– a simple HTTP server providing a web interface for viewing and editing RDF

– a fully customizable UI

– the beginnings of a peer-to-peer architecture for communication between different RDF databases

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