I am trying to inherit Qweb Report. I am successful in inserting static values inside my qweb Report but while i tried to add some computed module it didn't shown me the values. 

<odoo>

    <data>

        <template id="delivery_order_report" inherit_id="stock.report_picking">

                <xpath expr="//table[@id='mytable']" position="inside">

     <tfoot>

     <tr>

      <td>sum</td>

      <td><t t-esc="o.pack_operation_ids.pack_lot_ids.qty for o in docs])" /></td>

     </tr>

     </tfoot>

    </xpath>

  </template>

    </data>

</odoo>

you can see my code my inheriting qweb report of  model named as stock.report_picking. One interesting thing while i tried to fetch o.origin it shows me successfully . below is the code 

<odoo>

    <data>

        <template id="delivery_order_report" inherit_id="stock.report_picking">

                <xpath expr="//table[@id='mytable']" position="inside">

     <tfoot>

     <tr>

      <td>sum</td>

      <td><t t-esc="o.origin" /></td>

     </tr>

     </tfoot>

    </xpath>

  </template>

    </data>

</odoo>