withdrawing.py

class AccountInvoiceLine(osv.osv):
    _inherit = 'account.invoice.line'
    
    def _amount_line(self, cr, uid, ids, prop, unknow_none, unknow_dict):
        res = {}
        tax_obj = self.pool.get('account.tax')
        cur_obj = self.pool.get('res.currency')
        company_currency = self.pool['res.company'].browse(cr, uid, obj.company_id.id).currency_id.id
        for line in self.browse(cr, uid, ids):
            res[obj.id] = {'amount_tax': 0, 'price_subtotal': 0}
            price = line.price_unit * (1-(line.discount or 0.0)/100.0)
            cur = line.invoice_id.currency_id
            for tax_line in line.invoice_line_tax_id:
                base_amount = cur_obj.compute(cr, uid, cur.id, company_currency, line.price_subtotal * tax_line.base_sign, context={}, round=False)
                tax_amount = cur_obj.compute(cr, uid, cur.id, company_currency, tax_line.amount * tax_line.tax_sign, context={}, round=False)
                res[obj.id]['amount_tax'] = base_amount * tax_amount
                res[obj.id]['price_subtotal'] = el calculo de base del erp
        return res

    _columns = {
        'discount_value': fields.float('Discount Value'),
        'amount_tax': fields.function(_amount_line, string='Tax', type="float", digits_compute= dp.get_precision('Account'), store=True, multi='line',),
        'price_subtotal': fields.function(_amount_line, string='Amount', type="float", digits_compute= dp.get_precision('Account'), store=True, multi='line'),
        }