I don't find this convincing at all. > so that the addition works the same regardless of order, as exists for numeric types + is not commutative for lists, tu

I don't find this convincing at all. > so that the addition works the same regardless of order, as exists for numeric types + is not commutative for lists, tuples, or dicts. So the promotion process need not be commutative either. There is no good reason why list + tuple should be forbidden, or dict + items should be forbidden. a [op]= b is commonly and easily explained as "a = a [op] b, where a is mutated in place". Python should not break that explanation with mysterious inconsistencies.