Hello, Leonard. Shivoham's answer can serve as an suggestion for you. I have another way. Isn't your goal to discover the cheapest product? Assume you have attributes 'name,' 'price,' and 'vendor id' in the model 'product.'
You can 'group_by' all of its records by name, and see which record has the lowest 'price.'
You'll know which vendor provides that product thanks to the field 'vendor id.'

Thanks for the response Shivoham,

I have tried many for loops but they seem not work.

The below code is what I was trying to do.

def compute_cheapest(self):
    for rec in self:
        partner = rec.env['purchase.order'].search([('partner_id', '=', rec.partner_id.id)])
        for each_partner in partner:
            line = each_partner.order_line
            for item in line:
                if item.product_id._________________:


                    item.write({'quality': True})
                else:
                    item.write({'quality': False})

The place where I have put the dash is where I am stuck. How can I search in every vendor's oder_line and find similar products quoted by them. The cheapest price for that product I do the last code which I have written:

NB: I have set field quality to Boolean that's why in the end I am writting "True" or "False"

....
    item.write({'quality': True})
else:    item.write({'quality': False})

Or is there a better way to do it

Hello Leonard, 

You have to use loop in product vendor's list and take the minimum price from that and set it to order line.