In product module use onchange function and sets some value and return dictionary of value in on_client_changed function to first inherit the product module class like as product.template class and then use this function in .py file in openerp use this function given below,

def on_client_changed(self, cr, uid, ids, part, context=None):
        if not part:

            return {'value': {'partner_invoice_id': False, 'partner_shipping_id': False,  'payment_term': False, 'fiscal_position_id': False}}

        part = self.pool.get('res.partner').browse(cr, uid, part, context=context)
        addr = self.pool.get('res.partner').address_get(cr, uid, [part.id], ['delivery', 'invoice', 'contact', 'order'])
        pricelist = part.property_product_pricelist and part.property_product_pricelist.id or False
        payment_term = part.property_payment_term_id and part.property_payment_term_id.id or False
        fiscal_position_id = part.property_account_position_id and part.property_account_position_id.id or False
        dedicated_salesman = part.user_id and part.user_id.id or uid
        val = {
            'partner_invoice_id': addr['invoice'],
            'partner_shipping_id': addr['delivery'],
            'partner_order_id': addr['order'],
            'payment_term_id': payment_term,
            'fiscal_position_id': fiscal_position_id,
            'user_id': dedicated_salesman,
        }
        if pricelist:
            val['pricelist_id'] = pricelist
        return {'value': val}