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.
http://unicode.org/reports/tr11/ says:
"Ambiguous characters occur in East Asian legacy character sets as wide characters, but as narrow (i.e., normal-width) characters in non-East Asian usage."
So in practice applications can treat ambiguous characters as narrow by default, with a user setting to use legacy (wide) width.
As Tom pointed out there are also a bunch of zero width characters, and characters with special formatting like tab, soft hyphen, ...