'format excel reference notation'

def _get_ref(self):

    if self.pos2:
        return ('%s!$%s$%s:$%s$%s' % (self.sheet.title, get_column_letter((self.pos1[1] + 1)), (self.pos1[0] + 1), get_column_letter((self.pos2[1] + 1)), (self.pos2[0] + 1)))
    else:
        return ('%s!$%s$%s' % (self.sheet.title, get_column_letter((self.pos1[1] + 1)), (self.pos1[0] + 1)))
