[Python-ideas] Null coalescing operators

[Paul Moore]

On 21 September 2015 at 03:35, Mark E. Haase <mehaase at gmail.com> wrote:
> A) Is coalesce a useful feature? (And what are the use cases?)

There seem to be a few main use cases:

1. Dealing with functions that return a useful value or None to signal
"no value". I suspect the right answer here is actually to rewrite the
function to not do that in the first place. "Useful value or None"
seems like a reasonable example of an anti-pattern in Python.

2. People forgetting a return at the end of the function. In that
case, the error, while obscure, is reasonable, and should be fixed by
fixing the function, not by working around it in the caller.

3. Using a library (or other function outside your control) that uses
the "useful value or None" idiom. You have to make the best of a bad
job here, but writing an adapter function that hides the complexity
doesn't seem completely unreasonable. Nor does just putting the test
inline and accepting that you're dealing with a less than ideal API.

4. Any others? I can't think of anything.

Overall, I don't think coalesce is *that* useful, given that it seems
like it'd mainly be used in situations where I'd recommend a more
strategic fix to the code.

> B) If it is useful, is it important that it short circuits? (Put another
> way, could a function suffice?)

Short circuiting is important, but to me that simply implies that the
"useful value or None" approach is flawed *because* it needs
short-circuiting to manage. In lazy languages like Haskell, the Maybe
type is reasonable because short-circuiting is a natural consequence
of laziness, and so not a problem. In languages like C#, the use of
null as a sentinel probably goes back to C usage of NULL (i.e., it may
not be a good approach there either, but history and common practice
make it common enough that a fix is needed).

> C) If it should be an operator, is "??" an ugly spelling?
>
>     >>> retries = default ?? cls.DEFAULT

Arbitrary punctuation as operators is not natural in Python, something
like this should be a keyword IMO.

> D) If it should be an operator, are any keywords more aesthetically
> pleasing? (I expect zero support for adding a new keyword.)
>
>     >>> retries = default else cls.DEFAULT
>     >>> retries = try default or cls.DEFAULT
>     >>> retries = try default else cls.DEFAULT
>     >>> retries = try default, cls.DEFAULT
>     >>> retries = from default or cls.DEFAULT
>     >>> retries = from default else cls.DEFAULT
>     >>> retries = from default, cls.DEFAULT

Reusing existing keywords (specifically, all of the above) looks
clumsy and forced to me. I agree that proposals to add a new keyword
will probably never get off the ground, but none of the above
suggestions look reasonable to me, and I can't think of anything else
that does (particularly if you add "must be parseable" as a
restriction!)

Overall, I'm -0.5 on a "coalesce" operator. I can't see it having
sufficient value, and I can't think of a syntax I'd consider
justifying it. But if someone were to come up with a Guido-like
blindingly obvious way to spell the operation, I would be fine with
that (and may even start using it more often than I think).

Paul