3 ms·

You can hide the if statements with `bar = bar or Bar() `

Strilanc 5y ago
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....
- CraneWorm 5y ago
  bar = Bar() if bar is None else bar
- deleted 5y ago
  [deleted]
- monkeybutton 5y ago
  All of this is true. Its also weird to me that the evaluation of a boolean expression can return return a value that is neither True nor False (yes I know the value evaluates to true, that's why the trick works). Which is why I just do `if param is None:... ` and not worry about the few extra lines of code and move on. The trick does have the nice property of delaying execution for evaluating what the default parameter though. The only other way to do that would involve writing lambdas which is even more code? The sibling comment suggesting a package looks like even more code to write as a user. I think any really elegant fix would have to be done in the language itself.
  - hiptobecubic 5y ago
    It's not only not a trick, it's how the language is designed with the intention of doing it like this. To me it's like complaining that (++a) is a "trick" that lets you use secret temp values that are hard to understand. I mean.. _I guess_, but it's C so it seems like the problem is that you don't like C.

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....

CraneWorm 5y ago
bar = Bar() if bar is None else bar
deleted 5y ago
[deleted]
monkeybutton 5y ago
All of this is true. Its also weird to me that the evaluation of a boolean expression can return return a value that is neither True nor False (yes I know the value evaluates to true, that's why the trick works). Which is why I just do `if param is None:... ` and not worry about the few extra lines of code and move on. The trick does have the nice property of delaying execution for evaluating what the default parameter though. The only other way to do that would involve writing lambdas which is even more code? The sibling comment suggesting a package looks like even more code to write as a user. I think any really elegant fix would have to be done in the language itself.
- hiptobecubic 5y ago
  It's not only not a trick, it's how the language is designed with the intention of doing it like this. To me it's like complaining that (++a) is a "trick" that lets you use secret temp values that are hard to understand. I mean.. _I guess_, but it's C so it seems like the problem is that you don't like C.