I think you raise a good question, but Python doesn't have a traditional for loop. To do it in one loop, you'd either have to simulate a traditional for loop with something like range, or you'd have to build a c/zig/rust lib and use it with cffi (or whatever rust uses that I forgot what was named). Or you're going to do it the "pythonic" way and write two loops, probably with a generator. As far as micro optimisation I'd argue that it depends on what you want. Speed or stable memory consumption? The single loop will be faster (for the most part) but the flip side is that there is a limit on how big of a data set it can handle.

It's all theoretical though. On real world data sets that aren't small I don't see why you wouldn't hand these tasks off to C/Zig/Rust unless you're only running them once or twice.