Compute and return line number area width

def compute_width(self):
        """Compute and return line number area width"""
        if not self._enabled:
            return 0
        digits = 1
        maxb = max(1, self.editor.blockCount())
        while maxb >= 10:
            maxb /= 10
            digits += 1
        if self._margin:
            margin = 3+self.editor.fontMetrics().width('9'*digits)
        else:
            margin = 0
        return margin+self.get_markers_margin()