If this were the approach taken, it seems to me that there could not be any semantic change or side-effects for types that have compatible meaning for + and += (i.e. += is an in-place version of +).
Maybe I'm missing something here?
Only the fact that "there's nothing that guarantees" this, as Guido says. alist = alist + x only succeds if x is also a list, while alist += x succeeds also for tuples and other sequences, for example.
Personally, I don't think this would be a problem, but it's not my decision.
In the context of sum(), I think it would be nice to allow iterables to be added together: sum(['abc', range(3), ('do', 're', 'me')], []) This fits in well with the current thinking that the prohibition of adding sequences of unlike types be imposed only on operators and not on functions or methods. For instance, in sets.py, a|b requires both a and b to be Sets; however, a.union(b) allows b to be any iterable. The matches the distinction between list.__iadd__() and list.extend() where the former requires a list argument and the latter does not. Raymond Hettinger