[Python-ideas] Null coalescing operators

[Andrew Barnert]

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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