In Rust, for..in loops have the same problem: you don't know the type and shape of the thing you are iterating on before you get to the "in" part. "Luckily", it

In Rust, for..in loops have the same problem: you don't know the type and shape of the thing you are iterating on before you get to the "in" part. "Luckily", it does not allow you to call any methods on the object before you do. Similarly, Python list/dict/set comprehensions are a form of for-loop syntax sugar to easily create particular structure. One can use functools.maps to get exactly the same behavior of Rust example. If this was an all important text input microoptimization, we'd all be doing everything with pure functions like Lisp: yet somehow, functional languages are not the most popular even if they provide the highest syntax consistency.