unicodedata.east_asian_width().  You can still implement your own
simple width() function using it like this:
    def width(u):
        w = 0
        for c in unicodedata.normalize('NFC', u):
            cwidth = unicodedata.east_asian_width(c)
            if cwidth in ('W', 'F'): w += 2
            else: w += 1
        return w

document.

Documentation for the unicodedata module (massaged by Fred for minor
consistency issues).