Since we've already descended into anecdotal territory, lemme drop an anecdote. I had a dot-product function which turned out to be a hotspot in some numerical code. The original function looked something like: def dot(v1, v2): return sum(i * j for i, j in zip(v1, v2)) Very functional, very pretty, very straightforward. But it was slow, both in CPython and PyPy. A bit of bytecode analysis and profiling later, we real…
I think your original function was pure in the mathematical sense, but since you were operating on a data structure that is specifically 2 or 3 elements long, then you don't need to be that generic.
Just think of a vector of 3 elements as being one entity, and therefore it still obeys the 0, 1, Inf rule mentioned elsewhere in the comments.