> Martin, I think you meant to write "if w == 'A':".
> Some very common characters have ambiguous widths though (e.g. the Greek alphabet), so you can't just raise an error for them.

That's precisely why I don't think this should be in the library, but
in the application. Application developers who need that also need
to concern themselves with the border cases, and decide on how
they need to resolve them.