PythonのXML解析ライブラリの比較：LXML対ElementTree

lxml is faster and better for large files due to C libraries; 2. lxml supports advanced features like full XPath, XSLT, and schema validation; 3. ElementTree is built-in and dependency-free, while lxml requires installation; 4. lxml handles malformed XML more gracefully with better error diagnostics; use lxml for performance and advanced features, ElementTree for simplicity and portability.

When working with XML data in Python, two of the most commonly used libraries are lxml and xml.etree.ElementTree (often referred to as ElementTree). Both allow you to parse, modify, and generate XML, but they differ in performance, functionality, and ease of use. Here's a practical comparison to help you choose the right one for your use case.

1. Performance and Speed

lxml wins for large or complex XML files.

• lxml is built on top of the C libraries libxml2 and libxslt, making it significantly faster than the standard ElementTree, especially for parsing large files or performing XPath queries.
• ElementTree is written in pure Python (though the xml.etree.ElementTree module in modern Python uses a fast C implementation called xml.etree.cElementTree under the hood), but it's still generally slower than lxml.

✅ Use lxml if performance is critical (e.g., processing multi-megabyte XML files).

✅ Use ElementTree for smaller files or when speed isn’t a bottleneck.

2. API Compatibility and Ease of Use

They’re very similar — but lxml is more feature-rich.

• Both libraries use nearly identical APIs for basic operations like parsing, iterating, and modifying XML trees.
• Example: Parsing and iterating over elements looks almost the same:

# ElementTree
import xml.etree.ElementTree as ET
tree = ET.parse('data.xml')
for elem in tree.iter('item'):
    print(elem.text)

# lxml
from lxml import etree
tree = etree.parse('data.xml')
for elem in tree.iter('item'):
    print(elem.text)

• However, lxml supports more advanced features out of the box, such as:
  • Full XPath 1.0 support (ElementTree only supports a limited subset)
  • XSLT transformations
  • Better namespace handling
  • Support for DTD validation, XML Schema, and RelaxNG

✅ Use lxml if you need XPath, XSLT, or schema validation.

✅ Use ElementTree for simple XML tasks and when you want to avoid external dependencies.

3. Dependencies and Portability

ElementTree is included in the standard library.

• ElementTree comes with Python by default — no need to install anything.
• lxml is a third-party package. You need to install it via pip:

pip install lxml

• This makes ElementTree ideal for scripts that need to run in minimal environments or where installing C extensions is problematic.

✅ Use ElementTree for lightweight, dependency-free applications.

✅ Use lxml when you can manage dependencies and want more power.

4. Error Handling and Robustness

lxml is more forgiving and provides better diagnostics.

• lxml often handles malformed XML more gracefully (depending on parser settings).
• It provides more detailed error messages and better recovery options.
• You can customize the parser behavior easily:

from lxml import etree

# Handle broken XML with recover=True
parser = etree.XMLParser(recover=True)
tree = etree.parse('broken.xml', parser)

• ElementTree is stricter and may fail more abruptly on malformed input.

Summary: When to Use Which?

Bottom line:
If you're doing anything beyond basic XML manipulation — or care about speed — go with lxml.
If you want something simple, built-in, and "just works" for small tasks, ElementTree is perfectly fine.

And the best part? Since their APIs are so similar, switching between them later isn't too painful. You can prototype with ElementTree and upgrade to lxml when needed.

Basically, it comes down to: need more power? Use lxml. Just need to get the job done quickly and cleanly? ElementTree has your back.

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