If you want to extract the `product_qty` field from the `o.order_line` table in `py3o://function` and perform a sum calculation, you can use the following syntax:

pypy3o://function="sum([item.product_qty for item in o.order_line])"

In the above example, we used a list comprehension to iterate over each item in the o.order_line table and extract the value of the product_qty field for each item. Then, we passed these values as a list to the sum function for summation.

Please ensure that in your template, `o.order_line` is an iterable object, and each item within it has a `product_qty` field accessible.

If your needs are more complex and require more filtering conditions or logical operations, you can add corresponding expressions and conditions in the list comprehension to meet your requirements.

I hope this example helps you understand how to use list comprehensions in `py3o://function` to sum fields in the `o.order_line` table. If you have more questions or need further assistance, please provide more detailed code or context information.