I have a dict which contain a list product which will contain only one dict:

d = {"thickness": 90.0,"mass_surf": 37.8,"res_therm": 0.75,"codename": "codename1","category": "category1","sub_categories": ["sub_categories1"],"secondary_targets": ["secondary_targets1","secondary_targets2","secondary_targets3"],"product": [{"codename": "codename1","purpose": "purpose1","category": "category1","material": "material1"}]
}

I want to flat the product list of dict to obtain this:

d = {"thickness": 90.0,"mass_surf": 37.8,"res_therm": 0.75,"codename": "codename1","category": "category1","sub_categories": ["sub_categories1"],"secondary_targets": ["secondary_targets1","secondary_targets2","secondary_targets3"],"product.codename": "codename1","product.purpose": "purpose 1","product.purpose": "purpose1","product.category": "category1","product.material": "material1"
}

How can I do this?

product list will always contains only one item

You can flatten the internal dict and update it back to the original dict. Eg.,

product = d.pop("product")[0]
flattened = {f"product.{key}": value for key, value in product.items()}
d.update(flattened)

This should do the trick.