Does to_string cost much?

Which error should I raise out when I'm using Python?..

Anyone tells me which error should I throw out in Python?

Couldn't fuck py27 any more.
I've finished it in half an hour, however I've spent almost 2 hours and a half to detect out many differences between py27 and py3.

What sucks the most is that, what I spent most time on could be a little trick.

In py3, metaclass is a keyword when initilizing classes, while py2 use a static member named __metaclass__ to do the same things.

That's quite a bit awful.
I wrote multiple solutions in Python3.
First time when I submitted, an error occurred because of no __builtin__ in Python2.
The second time, no metaclass keywords when initializing a class.
The third time, no pattern matching like "args": (self, *args).

Em.... Sorry for my crazy words last night...
At that time, I didn't find that this is my kata, and I've done quite a lot things seems to be cheating.
When you replid, I'm so exhausted that I just thought you were talking about my cheating(I find the context can suite the both...
Now, as you see, I've deleted my old account :(