This isn't always safe to do. It can break duck typing, because not all falsy values are interchangeable in all situations. For example, consider how numpy methods often have an optional `out` parameter indicating where to write the output. So suppose you are writing a value producing method that takes an `out: Optional[List[int]] = None` to put the produced values into and you handle the `None` case with `out = out or []`. That's a bug! When the user tries to pass in a list to put things into, and that list happens to be empty, you will instead put outputs into a new list.

Anyways, my point is that I can write `if X is None: X = bla` and be confident it will be correct in almost all situations. For `X = X or Y` there are some nasty corner cases where the distinction between `[]` and `""` and `{}` is somehow important, or the distinction between `is` and `==` is important, or different falsy values have different annotations attached to them, or or or....