Hi Tony,

Use Related Field instead of making computing.

Try this:

in your .py file:

        product_categ_id = fields.Many2one(related = 'product_id.product_tmpl_id.categ_id', string='Product Category')

In your .xml file:

<odoo>

<data>

<record id="product_category_sale_line" model="ir.ui.view">

<field name="name">sale.orderline.inherited</field>

<field name="model">sale.order</field>

<field name="inherit_id" ref="sale.view_order_form"/>

<field name="arch" type="xml">

<xpath expr="//field[@name='invoice_line_ids']/tree/field[@name='product_id']" position="after">

<field name="product_categ_id"/>

</xpath>

</field>

</record>

</data>

</odoo>

Hope it Helps,

Regards,

Mayank Gosai

1) update __init__.py

2) update mainifest with xml and add 'purchase' in its data like 'data': ['purchase' ]

PY File:
from odoo import models, fields, api

            class ProductCategory(models.Model):

 _inherit= 'purchase.order.line'

product_categ_id = fields.Selection(string='Product Type',related='product_id.type')

XML File:

<odoo>

<data>

  <record id="purchase_order_line_form" model="ir.ui.view">

       <field name="name">purchase order line form</field>

       <field name="model">purchase.order</field>

       <field name="inherit_id" ref="purchase.purchase_order_form"/>

       <field name="arch" type="xml">

           <xpath expr="//form/sheet/notebook/page/field/tree/field[@name='product_id']" position="after">

               <field name="product_categ_id"/></xpath>

</field>

   </record>

  </data>

</odoo>

Thank you Mayank Gosai, for your reply. It really helped me to get the problem solved. Although what I wanted was to get the product type(ie, stockable,consumable,service)(Sorry, I mis-typed it to category and made the question ambiguous). I updated your code with some modifications to select the product type.
In the .py file

_inherit = 'sale.order.line'
product_categ_id = fields.Selection(related='product_id.product_tmpl_id.type', string='Category')

you can take one related field in sales order line object which belongs to product_id.category_id.

No need any function.