Return a week initialized from an ISO formatted string like "2011W08" or "2011-W08".

def fromstring(cls, isostring):
        """Return a week initialized from an ISO formatted string like "2011W08" or "2011-W08"."""
        if isinstance(isostring, basestring) and len(isostring) == 7 and isostring[4] == 'W':
           return cls(int(isostring[0:4]), int(isostring[5:7]))
        elif isinstance(isostring, basestring) and len(isostring) == 8 and isostring[4:6] == '-W':
           return cls(int(isostring[0:4]), int(isostring[6:8]))
        else:
            raise ValueError("Week.tostring argument must be on the form <yyyy>W<ww>; got %r" % (isostring,))