The expressivity gain depends on what you're left with without the feature. It was hard for me to imagine where the previous comment draws the line around iter

The expressivity gain depends on what you're left with without the feature. It was hard for me to imagine where the previous comment draws the line around iterators and generators. An optimizing compiler would need to know what actions are pure in order to rewrite code to be lazy. Python allows so much dynamism, I can only see that happening with a very sophisticated tracing JIT.