>> I sorely miss it in Python, JS and other languages. They keep me guessing whether a function will mutate the parent structure, or a local copy in those languages!

> Python at least is very clear about this ... everything, lists, class instances, dicts, tuples, strings, ints, floats ... are all passed by object reference. (Of course it's not relevant for tuples and scalars, which are immutable.)

Then let me just FTFY based on what you've said later:

Python will not be very clear about this ... everything, lists, class instances, dicts, tuples, strings, ints, floats, they all require the programmer to read and write documentation every time.