If it were possible that the interpreter iterating over a dictionary could cause the dictionary to change size then I suspect that this would be a rich source of bugs to mine. :-) 

It would be great if we could make it a documented, enforced invariant that the interpreter reading a dictionary never produces a side effect visible to the interpreter.

Re: "this specific statement already existed before my work."

Thanks for the historical note; the first thing I thought when I read this line was "that's got to be a copy-paste left over from an earlier version of the algorithm."