Good to know thanks!

I think it would be more efficient to save it in a variable rather than calling it multiple times. It's not a big deal here because the lists are small (and maybe you have memory problems if the number is big? idk).
Basically ppl on CW just try to write all their code on one line regardless of efficency (as long as it passes all tests).

Hi, I see a lot of solutions on codewars that use len(x) (or min, lower, count...) in loops or throughout the code rather than doing it once and saving the result in a reusable variable, which I would think is more efficient. Is it not? Why do you use it this way? Readability? Code minimisation?