[Python-ideas] Null coalescing operators

[MRAB]

On 2015-09-19 02:10, Andrew Barnert via Python-ideas 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.
>
I see it as "spam? doing "Maybe(spam)" and then attribute access
checking returning None if the wrapped object is None and getting the
attribute from it if not.

I think that the optimiser could probably avoid the use of Maybe in
cases like "spam?.cheese".