Yeah could have simplified even more like that, to me kata 7 you could make it straightforward and easy to read. If you interested on golf solution, yes, we could use this or even better came out with an equation for the number lines.

Anyways, thanks for the note, Hope keep it here for future reference.

Sorry for coming with a native method for a performance tag problem. I am just trying it at first and it works; As a saying "If it works, Don't Touch It!".

Anyways, Sorry for the inconvenience.

Lol, I'm really thought that the a.extend(b) as a requirement on this kata. I Think I Overthink Things.

@FArekkusu comments help me alot! Thanks!

enumerate(iterable, start=0)
arr[0] is pass to start with arr[0] instead of 0.

I hope this help, but if not you can refer this:
https://docs.python.org/3/library/functions.html#enumerate

Return list of birds that not in the list of geese.

is it better if we put the "conversion" var inside function or outside as class var?