Except that nothing is passed by reference (or everything is, depending on how you look at it). It's just that some objects have methods which can modify state (like list, dict) and others don't (like int, str). Ned Batchelder did a good presentation on this many years ago which is still probably the best I've seen for explaining how it works to people coming from other languages - https://nedbatchelder.com/text/names1/names1.html - where the names passed to a function act just like regular assignment.