从stock.picking模型中获取产品税

Question

我想从stock.picking模型计算所包含的总税款。这是我的代码：

@api.multi
@api.depends('move_lines', 'move_lines.product_qty',
             'move_lines.product_uos_qty')
def _amount_all(self):
    print '_amount_all dans StockPicking'
    for picking in self:
        picking.amount_untaxed = 0.0
        picking.amount_total = 0.0
        val2 = val1 = val = 0.0
        for line in picking.move_lines:
            print  line.product_id.id
            # using to get product taxes
            taxes=self.env['account.tax'].compute_all(line.price_unit,line.product_uom_qty,line.product_id.id)
            print taxes
            val1 += taxes['total']
            val += taxes['total_included']
            for tax in taxes['taxes']:
                val2 += tax.get('amount', 0.0)
        picking.amount_untaxed = val1
        picking.amount_tax = val2
        picking.amount_total = val

但是行taxes=self.env['account.tax'].compute_all(line.price_unit,line.product_uom_qty,line.product_id.id)始终返回空税。

注意：从报价单中我可以获得税额。