Odoo,Qweb,Reportレポートの外部メソッド参照の問題

import time
from openerp.osv import osv
from openerp.report import report_sxw


class stock_picking_out(report_sxw.rml_parse):
    def __init__(self, cr, uid, name, context):
        super(stock_picking_out, self).__init__(cr, uid, name, context=context)
        self.localcontext.update({
            'time': time,
            'get_partner_product': self._get_partner_product,
        })

    # todo  :
    def _get_partner_product(self, partner, product_id):
        sql = """SELECT cp.partner_default_code FROM customer_product cp
                LEFT JOIN res_partner rp ON rp.id = cp.partner_id
                WHERE cp.product_id = %s AND cp.partner_id = %s"""
        self.cr.execute(sql, (product_id, partner))
        sql_data = self.cr.fetchall()
        if len(sql_data) > 0:
            for obj in sql_data:
                partner_default_code = obj[0]
            return partner_default_code
        return ''


class report_stockpickingout(osv.AbstractModel):
    _name = 'report.report_stock.report_stockpickingout'
    _inherit = 'report.abstract_report'
    _template = 'report_stock.report_stockpickingout'
    _wrapped_report_class = stock_picking_out

<td class="text-left">
    <span t-esc="get_partner_product(doc.partner_id.id,l.product_id.id)"/>
td>

QWebException: "'NoneType' object is not callable" while evaluating
'get_partner_product(doc.partner_id.id,l.product_id.id)'

class report_stockpickingout(osv.AbstractModel):
    _name = 'report.report_stock.report_stockpickingout'