Fast XML parsing using Expat in Python

Python's xml.parsers.expat module provides fast XML parsing using the Expat library. It is a non-validating XML parser that creates an XML parser object and captures XML elements through various handler functions. This event-driven approach is memory-efficient and suitable for processing large XML files.

How Expat Parser Works

The Expat parser uses three main handler functions ?

• StartElementHandler − Called when an opening tag is encountered
• EndElementHandler − Called when a closing tag is encountered
• CharacterDataHandler − Called when character data between tags is found

Example

Here's how to parse XML data using Expat with custom handler functions ?

import xml.parsers.expat

# Capture the first element
def first_element(tag, attrs):
    print('first element:', tag, attrs)

# Capture the last element
def last_element(tag):
    print('last element:', tag)

# Capture the character Data
def character_value(value):
    print('Character value:', repr(value))

parser_expat = xml.parsers.expat.ParserCreate()
parser_expat.StartElementHandler = first_element
parser_expat.EndElementHandler = last_element
parser_expat.CharacterDataHandler = character_value

parser_expat.Parse("""<?xml version="1.0"?>
<parent student_rollno="15">
<child1 Student_name="Krishna"> Strive for progress, not perfection</child1>
<child2 student_name="vamsi"> There are no shortcuts to any place worth going</child2>
</parent>""", 1)

The output of the above code is ?

first element: parent {'student_rollno': '15'}
Character value: '\n'
first element: child1 {'Student_name': 'Krishna'}
Character value: ' Strive for progress, not perfection'
last element: child1
Character value: '\n'
first element: child2 {'student_name': 'vamsi'}
Character value: ' There are no shortcuts to any place worth going'
last element: child2
Character value: '\n'
last element: parent

Key Points

• The parser processes XML sequentially, calling handlers as it encounters elements
• Whitespace characters (like newlines) are captured as character data
• Attributes are passed as dictionaries to the start element handler
• The final parameter 1 in Parse() indicates this is the final chunk of data

Conclusion

Expat parser is ideal for fast, memory-efficient XML processing in Python. It uses event-driven parsing with handler functions to process XML elements, attributes, and character data sequentially.