XML Format Guide: XPath, XSLT, and Python Processing

What Is XML?

XML (eXtensible Markup Language) is an extensible markup language defined by the W3C in 1998. Unlike HTML, XML lets you create custom tags to describe any data structure.

Standard in: SOAP services, configurations (Maven, Spring, Android), .docx/.xlsx internals, RSS/Atom feeds, industrial data (HL7, FIXML).

XML Syntax

<?xml version="1.0" encoding="UTF-8"?>
<catalog>
  <product id="101" category="electronics">
    <name>Laptop Pro X</name>
    <price currency="USD">1299.99</price>
    <description>
      <![CDATA[With <special> characters & symbols]]>
    </description>
    <available>true</available>
  </product>
</catalog>

Rules: one root element, tags must close, attributes in double quotes, case-sensitive. Special chars: & (&), < (<), > (>), " (").

XPath Navigation

import lxml.etree as ET

xml = b"""
<catalog>
  <product id="1"><name>Apple</name><price>1.20</price></product>
  <product id="2"><name>Orange</name><price>0.80</price></product>
</catalog>"""

tree = ET.fromstring(xml)
expensive = tree.xpath("//product[price > 1]")
for p in expensive:
    print(p.find("name").text)

prod = tree.xpath("//product[@id='1']")[0]
print(prod.find("name").text)  # Apple

Process XML in Python

import xml.etree.ElementTree as ET

tree = ET.parse("catalog.xml")
root = tree.getroot()

for product in root.findall("product"):
    name = product.find("name").text
    price = float(product.find("price").text)
    print(f"{name}: USD {price}")

# 10% price increase
for price_el in root.iter("price"):
    price_el.text = str(round(float(price_el.text) * 1.1, 2))

# Add product
new = ET.SubElement(root, "product")
ET.SubElement(new, "name").text = "Grape"
ET.SubElement(new, "price").text = "2.00"

ET.indent(tree, space="  ")
tree.write("catalog_updated.xml", encoding="UTF-8", xml_declaration=True)

Convert XML to JSON

import xmltodict, json

with open("catalog.xml", "rb") as f:
    doc = xmltodict.parse(f)

print(json.dumps(doc, ensure_ascii=False, indent=2))

Validate with XSD

from lxml import etree

schema = etree.XMLSchema(etree.parse("catalog.xsd"))
doc = etree.parse("catalog.xml")
if schema.validate(doc):
    print("XML is valid")
else:
    for err in schema.error_log:
        print(f"Error: {err.message}")

XSLT: Transform XML to HTML

from lxml import etree

dom = etree.parse("catalog.xml")
xslt = etree.parse("catalog-html.xsl")
html = etree.XSLT(xslt)(dom)
with open("catalog.html", "wb") as f:
    f.write(bytes(html))

XML vs JSON

Conclusion

XML remains essential in legacy systems, SOAP, and office documents. For new REST projects, JSON is preferable. Mastering XPath and Python's lxml covers 95% of XML use cases.

Related conversions

Document conversions that follow this topic naturally:

• PDF to DOCX
• DOCX to PDF
• MD to PDF
• HTML to PDF
• ODT to PDF
• PDF to TXT