lxml

lxml is the go-to library when you need serious XML/XSLT processing in Python. The libxml2 backend makes it significantly faster than pure-Python alternatives, and the ElementTree-compatible API feels natural. I've used it extensively for parsing complex XML schemas, SOAP integrations, and document transformations where performance matters.

The security story requires careful attention. XML External Entity (XXE) attacks are a constant concern—you must explicitly configure XMLParser with resolve_entities=False and no_network=True when handling untrusted input. The defaults have improved over time, but it's not fully secure-by-default. Error messages can leak filesystem paths during parsing failures, which I've had to sanitize in production logs. The C extension dependency means you're trusting the libxml2/libxslt supply chain, and building from source requires development headers.

Documentation is comprehensive with good security sections, but the onus is on developers to read and apply them. Input validation helpers like defusedxml integration would be welcome. For trusted internal XML processing, it's excellent. For user-facing parsers, you need defensive configuration and thorough testing.

lxml has been my go-to for XML and HTML processing for years, and it consistently delivers. The ElementTree API is intuitive and Pythonic, making simple tasks straightforward while XPath and XSLT support handles complex scenarios beautifully. Performance is outstanding compared to pure Python alternatives, which matters when processing large documents or doing bulk operations.

The documentation is comprehensive with practical examples that you can actually copy and adapt. The tutorial walks you through common patterns step-by-step, and the API reference is detailed without being overwhelming. Error messages are generally helpful, though encoding issues can sometimes produce cryptic libxml2 errors that require digging.

Debugging is straightforward thanks to clean stack traces and the ability to pretty-print XML at any point. The cssselect integration for HTML parsing is a killer feature that makes web scraping much more pleasant than dealing with raw XPath. Community support is solid - most common issues have Stack Overflow answers, and the mailing list archives are searchable when you hit edge cases.

lxml has been my go-to for XML processing across multiple production projects. The library beautifully combines the ElementTree API that most Python developers know with the raw power of libxml2. XPath and XSLT support is exceptional - complex queries that would be painful with pure ElementTree become straightforward. The `etree.tostring()` and parsing methods handle encoding gracefully, and namespace handling actually makes sense once you understand the `nsmap` parameter.

Error messages are generally helpful, pointing to line numbers in malformed XML and providing context. The documentation is comprehensive with a tutorial that covers 80% of common use cases. I particularly appreciate the detailed examples for XPath expressions and CSS selectors via `cssselect`. Debugging is straightforward since you can inspect the tree structure intuitively.

The main learning curve is installation-related (C dependencies) and understanding when to use lxml vs standard library xml.etree. Performance is excellent even with large documents. Stack Overflow has extensive coverage of common issues, and the official docs answer most questions before you need to search elsewhere.