> The current design of Python guarantees that an object always gets a setattr or setitem when one of its elements is assigned to. That's an important property, for the reasons I suggested above. So any change would have to preserve that property. And skipping assignment when __iadd__ returns self would not preserve that property. So it's not just backward-incompatible, it's bad.

--> some_var = ([1], 'abc')
--> tmp = some_var[0]
--> tmp += [2, 3]
--> some_var
([1, 2, 3], 'abc')

In that example, 'some_var' is modified without its __setitem__ ever being called.