It's because of Python's distinction (enforced mainly by convention) between references and value types. If you are using a value type, `__iadd__`[1] can retur

It's because of Python's distinction (enforced mainly by convention) between references and value types. If you are using a value type, `__iadd__`[1] can return a new value. So: x = 5 y = x x += 3 y is not x and y == 5 But if you're using a reference type, it is allowed to mutate the reference and return the original reference. x = [1, 2] y = x x += [3, 4] y is x and y == [1, 2, 3, 4] [1]: https://docs.python.org/3/reference/datamodel.html#object.__iadd__ https://docs.python.org/3/reference/datamodel.html#object.__...