Hi Ayman, as you are already inheriting the product.template object then,again there is no need of defining the _name for your inherited model, so you need to remove this : _name = "product.template"

you add file name in to __init__.py

and remove _name

restart server and update module and reload page.

then it's work i am 100% sure. 

Thank you.

Try this..

class product_template(osv.osv):
    _inherit =  "product.template"   

    _columns = { 'description': fields.text('Description', translate=True,help="A precise description of the   Product, used                                 only for internal information purposes. Extended!!!"),    }

Note: No need to write the name of the model (_name = "product.template") while inheriting a model .. !

Hope this helps !!!

If you create a new field in your inheritance class, does it appear in the fields list of the model product.template ?

What if you try to use this syntax?

description = fields.Text(string="Description", translate=True,help="A precise description of the Product, used only for internal information purposes. Extended!!!")

In Odoo/OpenERP we can inherit or use existing modules object/class/model and views. We can also inherit single field of existing modules. The question is why we need such inheritance.

The purpose of inheritance or why we need inheritance is given below:

• To change attributes of some fields which exists on existing/custom model (e.g. making fields readonly,invisible)

• To add/modify/delete old or new fields in existing/custom model (e.g. Product, Sales, HR, Fleet Management, Attendance modules model etc)

• We can also add buttons in already existing/custom model (form and tree) view by using inheritance

Get detailed description about inheritance: http://learnopenerp.blogspot.com/2018/01/inheritance-in-models-and-views.html

remove _name and put only _inherit . _inherit should contain the table name