If, however, you are reading a Python tutorial (general, not modding for Civ), and are attempting to use native capability to directly read XML data, then things are more complicated, and the game isn't completely set up to want to allow such things. You can probably pull it off just fine, but there will be a LOT of legwork to set up a proper python environment, file interfaces and whatnot, and then a significant performance hit while running the game.
Civ is built so that XML is read by the DLL when you first launch the program, then never touched again. Python asks for data from the DLL, and cannot directly access the XML information at any point. New fields added to the XML needs to be set up in the DLL to work.