ElementTree and lxml:
Start here if you want to write new code for XML processing
lxml tutorial: http://lxml.de/tutorial.html
Dive into Python 3, chapter on XML processing: https://www.cmi.ac.in/~madhavan/courses/prog2-2012/docs/diveintopython3/xml.html
Start here if you need to maintain existing DOM/SAX code that uses the xml.dom and xml.sax packages, or want to process XML using these packages
Note: some of these also describe the use of PyXML, which is an outdated XML package that is no longer maintained and should not be used in new code.
Python XML Tutorial: https://www.tutorialspoint.com/python/python_xml_processing.htm