[Python-ideas] Null coalescing operators

[Ryan Gonzalez]

This is likely going to get shot down quickly...

I know CoffeeScript is not regarded too well in this community (well, at least based on Guido's remarks on parsing it), but what if x? was shorthand for x is None? In CS, it's called the existential operator.

On September 18, 2015 10:30:48 PM CDT, Andrew Barnert via Python-ideas <python-ideas at python.org> wrote:
>On Sep 18, 2015, at 19:06, Mark E. Haase <mehaase at gmail.com> wrote:
>> 
>> Andrew, I really like that idea. Turning back to the null coalescing
>operator (spelled ?? in other languages), how do you think that fits
>in? 
>> 
>> Consider this syntax:
>> 
>> >>> None? or 1
>
>I don't think there's any easy way to make "spam? or 1" work any better
>than "spam or 1" already does, partly for the reasons you give below,
>but also because it doesn't seem to fit the design in any obvious way.
>
>I guess that means postix ? doesn't quite magically solve everything...
>
>> This also doesn't work quite right. If both operands are None, we
>want the expression to evaluate to None, not NoneQuestion. Should null
>coalescing be a separate operator? And if so, are "?" and "??" too
>similar?
>
>As MRAB pointed out, there seem to be good reasons to let spam?? mean
>the same thing as spam? (and that follows automatically from the
>simplest possible definition, the one I gave above). So I think "spam
>?? eggs" is ambiguous between the postfix operator and the infix
>operator without lookahead, at least to a human, and possibly to the
>compiler as well.
>
>I suppose ?: as in ColdFusion might work, but (a) ewwww, (b) it
>regularly confuses novices to CF, and (c) it's impossible to search
>for, because ?: no matter how you quote it gets you the C ternary
>operator....
>
>> Can anybody think of realistic use cases for overriding a magic
>method for the "?" operator? I would like to include such use cases in
>a PEP. One possible use case: being able to coalesce empty strings.
>> 
>> >>> s1 = MyString('')
>> >>> s2 = MyString('foobar')
>> >>> s1? or s2
>> MyString('foobar')
>
>This seems like a bad idea. Empty strings are already falsey. If you
>want this behavior, why not just use "s1 or s2", which already works,
>and for obvious reasons?
>
>>> On Fri, Sep 18, 2015 at 9:10 PM, Andrew Barnert via Python-ideas
><python-ideas at python.org> wrote:
>>> On Sep 18, 2015, at 18:00, Chris Angelico <rosuav at gmail.com> wrote:
>>> >
>>> >> On Sat, Sep 19, 2015 at 10:49 AM, Andrew Barnert
><abarnert at yahoo.com> wrote:
>>> >> Obviously "spam?" returns something with a __getattr__ method
>that just passes through to spam.__getattr__, except that on NoneType
>it returns something with a __getattr__ that always returns None. That
>solves the eggs case.
>>> >>
>>> >> Next, "spam?.cheese?" returns something with a __call__ method
>that just passed through to spam?.cheese.__call__, except that on
>NoneType it returns something with a __call__ that always returns None.
>That solves the cheese case.
>>> >
>>> > Hang on, how do you do this? How does the operator know the
>difference
>>> > between "spam?", which for None has to have __getattr__ return
>None,
>>> > and "spam?.cheese?" that returns (lambda: None)?
>>> 
>>> >>> spam
>>> None
>>> >>> spam?
>>> NoneQuestion
>>> >>> spam?.cheese
>>> None
>>> >>> spam?.cheese?
>>> NoneQuestion
>>> >>> spam?.cheese?()
>>> None
>>> 
>>> All you need to make this work is:
>>> 
>>> * "spam?" returns NoneQuestion if spam is None else spam
>>> * NoneQuestion.__getattr__(self, *args, **kw) returns None.
>>> * NoneQuestion.__call__(self, *args, **kw) returns None.
>>> 
>>> Optionally, you can add more None-returning methods to NoneQuestion.
>Also, whether NoneQuestion is a singleton, has an accessible name, etc.
>are all bikesheddable.
>>> 
>>> I think it's obvious what happens is "spam" is not None and
>"spam.cheese" is, or of both are None, but if not, I can work them
>through as well.
>>> 
>>> 
>>> > ChrisA
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>> 
>> 
>> 
>> -- 
>> Mark E. Haase
>> 202-815-0201
>
>
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