Hi,

Please try it like this.

import xmlrpc.client

import base64

common = xmlrpc.client.ServerProxy('http://localhost:9016/xmlrpc/2/common')

uid = common.authenticate('dbname', 'username', 'key', {})

models = xmlrpc.client.ServerProxy('http://localhost:9016/xmlrpc/2/object')

with open("test.pdf", "rb") as f:

    encoded_file = base64.b64encode(f.read()).decode("utf-8")

attachment_id = models.execute_kw(

    'dbname', 'username', 'key',

    'ir.attachment', 'create',

    [{

        'name': 'Sample Attachment',

        'type': 'binary',

        'datas': encoded_file,

        'res_model': 'sale.order', <--- your model

        'res_id': 324324,  # <--- your line ID

        'mimetype': 'application/pdf',

        'store_fname': 'test.pdf',

        'public': True, # <----- optional for public access

    }]

)

print("Attachment created with ID:", attachment_id)

Now to ensure this attachment is created you can check by going to url '/web/content/{attachment_id}.

Hope it helps