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XML

Full form - eXtensible Markup Language Structured, verbose, schema validation. Enterprise standard.

Syntax​

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <app>MyApp</app>
    <version>1.0.0</version>
    <debug>true</debug>
    <port>8080</port>

    <database>
        <host>localhost</host>
        <port>5432</port>
        <name>mydb</name>
    </database>

    <features>
        <feature>auth</feature>
        <feature>logging</feature>
        <feature>cache</feature>
    </features>

    <!-- Comment -->
    <timeout>30</timeout>
</configuration>

Attributes vs Elements​

<!-- As attributes -->
<server host="localhost" port="8080" debug="true"/>

<!-- As elements -->
<server>
    <host>localhost</host>
    <port>8080</port>
    <debug>true</debug>
</server>

Best practice: Use elements (more structured, easier to parse).

Special Features​

CDATA (for special characters)​

<query>
    <![CDATA[
        SELECT * FROM users WHERE name = 'O'Brien'
    ]]>
</query>

Namespaces (avoid unless required)​

<config xmlns:db="http://example.com/database">
    <db:connection>localhost:5432</db:connection>
</config>

Pros ✅​

  • Self-documenting
  • Schema validation (XSD)
  • Comments allowed
  • Namespaces for organization
  • Industry standard (enterprise)

Cons ❌​

  • Verbose (much larger files)
  • Slow to parse
  • Complex syntax
  • Not human-friendly
  • Overkill for simple configs

When to Use​

  • Enterprise applications
  • Legacy system integration
  • Complex data + validation required
  • SOAP web services
  • When schema (XSD) is critical

Python Usage​

import xml.etree.ElementTree as ET

# Read
tree = ET.parse('config.xml')
root = tree.getroot()

# Access values
app = root.find('app').text
host = root.find('database/host').text

# Access attributes
port = root.find('server').get('port')

# Write
root = ET.Element('configuration')
ET.SubElement(root, 'app').text = 'MyApp'
ET.SubElement(root, 'port').text = '8080'

tree = ET.ElementTree(root)
tree.write('config.xml', encoding='utf-8', xml_declaration=True)

Iterate Elements​

# Loop through features
for feature in root.findall('features/feature'):
    print(feature.text)

# Loop with attributes
for server in root.findall('server'):
    name = server.get('name')
    host = server.get('host')
    print(f"{name}: {host}")

Common Mistakes​

❌ Missing XML declaration

<configuration>
    ...
</configuration>

✅ Always include declaration

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    ...
</configuration>

❌ Forgetting to close tags

<host>localhost

✅ Always close tags

<host>localhost</host>

Validation (XSD)​

# Read with validation
from lxml import etree

schema = etree.XMLSchema(file='schema.xsd')
doc = etree.parse('config.xml')

if schema.validate(doc):
    print("Valid!")
else:
    print(schema.error_log)

Tools​

# Pretty print
xmllint --format config.xml

# Validate against schema
xmllint --schema schema.xsd config.xml

# Extract value
xmllint --xpath '//database/host/text()' config.xml

Size Comparison​

Same config in 3 formats:

JSON (0.1 KB): {"host":"localhost","port":5432}

YAML (0.15 KB):

host: localhost
port: 5432

XML (0.3 KB):

<?xml version="1.0"?>
<config>
    <host>localhost</host>
    <port>5432</port>
</config>

Verdict: XML is 3x larger for same data.