Why inlining _len(), I don't have seen performance issues and it's less readable (less pythonic)

How do you know where to break once you have the whole value?

Hello I was reading too fast. In my version there's the _wide boolean function.
So here width += _wide(char) + 1

And _len is just return sum(2 if _wide(char) else 1 for char in text) with no performance issues

More pythonic, DRY.

I won't bet on the performances, calling _len from _slice adds two functions calls per character (one to _len and one to sum). In one case I'm doing it on a character, and in the other case in a whole string. Yes I could also factorize this ternary to a third function, but I don't find it more readable.

Why note use enumerate(), it's less readable (less pythonic)

Because it does not works with enumerate as the last incrementation were not done. I do not remember which case exactly but if you run the unit test you'll spot it easily, it was failing, I'll do if needed but can't right now.

Interested in that, the code was strongly tested for txt2tags and don't catch this problem.

Your initial implementation was working thanks to your if cjk_len(text) <= index: return text, '' fixing the special case explicitly, I may have tried to avoid it.

"Explicit is better than implicit." but the more important is that both solutions are correct.

I have done a more complex solution:

elif self.width == 1 and (sum(self._width(chunk) for chunk in chunks) >
                              sum(len(chunk) for chunk in chunks)):

It throws the exception earlier, but it's probably not absolutely necessary.