Sure, it makes sense if you find something like `len_x` or `x_count` significantly less readable than `len(x)` and if you don't have to worry about resources. Can't say I've been there though.

I did a quick test of this `reverse` function (which probably shouldn't exist in the first place) and, unsurprisingly, it became ~30% faster when `len(arr)` was only called once.