d>''<b

is a double thumbs-up!

@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.