And another bit of info: it's much harder to reliably track that, because as soon as the list (for example) escapes, its elements could have any type and it cou

And another bit of info: it's much harder to reliably track that, because as soon as the list (for example) escapes, its elements could have any type and it could be any size. PyPy has storage strategies and speculative typing (with guards and deoptimization) that help with this kind of thing.