__init__.py

from . import models

from odoo import api, SUPERUSER_ID


def pre_init_product_code(cr):
    env = api.Environment(cr, SUPERUSER_ID, {})

    cr.execute(
        """
        SELECT product_tmpl_id from product_product
        WHERE default_code is NULL
        OR LENGTH(default_code) = 0
        GROUP BY product_tmpl_id
        HAVING COUNT(product_tmpl_id) = 1"""
    )
    product_template_ids = [x[0] for x in cr.fetchall()]
    cr.execute(
        """UPDATE product_product
        SET default_code = 'DEFAULT' || nextval('ir_default_id_seq')
        WHERE default_code is NULL
        OR LENGTH(default_code) = 0"""
    )

    env["product.template"].browse(product_template_ids)._compute_default_code()

    return True