Thanks for your answer and examples The question arised while trying to modify "create" method to add automatically an EAN13 code when a product is created. As 'ean13' is a field of 'product.product' initially you start modifying 'create' method for 'product.product', but the EAN13 code is actually managed and populated via 'create' method in 'product.template' (so 'create' method for 'product.product' has no visibility on the EAN13 entered by the user). Once business logic is moved into 'create' 'product.template' instead of 'product.product', it is easy to complete the task.

Now is clear.
Since I prefer manage Odoo externally trough Python to have a full control I don't have a deep knowledge of internal coding. Anyway, did you try to inherit the table adding a fields.related or compiling a simple fields.char it via on_change?

I am not sure if on_change applies here, I have used once for on-the-fly calculations in a form. I guess there might be a way to trigger some code when a new product is added, which might fit in this scenario. Regarding the approach I was looking for, as the field is already there (ean13 in product_product table), it was just a matter of doing something if user has not entered EAN13, so I think that the "right" approach is to modify the method create adding the required logic. There might be other or better approaches though; I am far from having a sound knowledge of Odoo.