@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).

you mostly cannot trust the "best practices" solutions to actually be best practices (because of a lot of biases, and mostly the fact that most people voting actually do not know what they are doing when voting and just up vote "something that looks cool").

ok thankyou. but i confuce how wrongly written can be best practice.

a class var would be better, yes.

the code is "wrongly" written, yes. It should be cls. But since this argument isn't used at all in the code, one can call it whatever one wants (even inappropriate stuff, like in the present situation)

why when use @classmethod decorator still use self not cls?

ok, thanks

When you post here a solution like you did and don't use spoiler flag, your post is visible on the homepage for anyone to see.

what the fuction?

Please use spoiler flag next time.