Thank you!

List lookup is inefficient - O(n) - whereas sets and dicts are O(1).

I do not believe that so many people think same.
I wasted half a day for a shitty-working-dirty solution, was so happy, and saw THAT.

Rough world

LMAO!!!

python programmers trying their absolute hardest to write all their code in 1 line

This solution is absolutely this more impressive solution i ever see to any katas!

I was wondering how I could get this type of solution all on one line. Great code and very readable

You are not alone....

@cahyareza

if x < len(s) - 1
is the same as:
if len(s) - x > 1
meaning that the cursor (x) has to be before (<) the last position (len(s) - 1), in other words at the penultimate position at most, and in this case you want a slice of 2 (s[x:x+2]);
otherwise, if your cursor is at the last position, you want the last character plus an underscore (s[-1] + "_").
You are safe since you're stepping 2 by 2 (in range(0, len(s), >>2<<)).

Also remember that positions start with 0, so each one is "position - 1" (the first one is "1 - 1 = 0", the second one is "2 - 1 = 1", ..., hence the last one is len(s) - 1).

I actually love this one as it fits better in a functional programming mindset.

I am impressed with the simplicity of this solution. It is often the simple simple that is most effective.
I still have a lot to learn!