Good explanation of the Python object model and how parameters are passed to functions. This can look rather surprising or confusing if you come from another language. Always keep in mind: mutability and the fact that variable name are just labels play a big role in this. That means you might need to copy explicitly in the case of mutable objects... which makes the performance burden explicit as well (and this means you need to pick between shallow or deep copying).

PS: I really mean "label" here (like in the post), it's a bit different from "pointer" since you don't get the pointer geometry (you can only reassign). From the C++ perspective I'd say it behaves as if all variables were "reassignable non-const references" only, something like that.