[Python-ideas] Null coalescing operator

[Guido van Rossum]

So you're offering `NoneCoalesce(x).bar` as less-ugly alternative to
`x?.bar`... Color me unconvinced.

On Sat, Sep 10, 2016 at 4:06 PM, David Mertz <mertz at gnosis.cx> wrote:
> Actually, I guess the example I liked was from the year ago discussion.  And
> it didn't do *exactly* what I think a wrapper should.  What I'd want would
> be like this:
>
> class NoneCoalesce(object):
>     "Standard operations on object for 'is not None'"
>     def __init__(self, obj):
>         self.obj = obj
>
>     def __getattr__(self, name):
>         try:
>             return getattr(self.obj, name)
>         except AttributeError:
>             return NoneCoalesce(None)
>
>     def __getitem__(self, item):
>         try:
>             return self.obj[item]
>         except (TypeError, KeyError):
>             return NoneCoalesce(None)
>
>     def __call__(self, *args, **kwds):
>         try:
>             return self.obj(*args, **kwds)
>         except TypeError:
>             return NoneCoalesce(None)
>
>     def __bool__(self):
>         return self.obj is not None
>
>     def __repr__(self):
>         return "NoneCoalesce[%r]" % self.obj
>
>     def __str__(self):
>         return "NoneCoalesce[%r]" % self.obj
>
>     def __len__(self):
>         try:
>             return len(self.obj)
>         except TypeError:
>             return 0
>
>
> Then we might use it similar to this:
>
>>>> from boltons.dictutils import OrderedMultiDict
>>>> from NoneCoalesce import NoneCoalesce
>>>> omd = OrderedMultiDict()
>>>> omd['a'] = 1
>>>> omd['b'] = 2
>>>> omd.add('a', 3)
>>>> nc = NoneCoalesce(omd)
>>>> nc or "Spanish Inquisition"
> Out[8]: NoneCoalesce[OrderedMultiDict([('a', 1), ('b', 2), ('a', 3)])]
>>>> nc.spam or "Spam"
> Out[9]: 'Spam'
>>>> nc['nope'].bar.baz()
> Out[10]: NoneCoalesce[None]
>>>> nc['a']
> Out[11]: 3
>>>> nc.getlist('a')
> Out[12]: [1, 3]
>
>
> Nothing special about boltons' OrderedMultiDict here, just something I've
> been playing with that has some distinctive methods.
>
> The idea is that we can easily have both "regular" behavior and None
> coalescing just by wrapping any objects in a utility class... and WITHOUT
> adding ugly syntax.  I might have missed some corners where we would want
> behavior wrapped, but those shouldn't be that hard to add in principle.
>
> On Sat, Sep 10, 2016 at 3:21 PM, David Mertz <mertz at gnosis.cx> wrote:
>>
>> I find the '?.' syntax very ugly, much more so in the examples of chained
>> attributes.
>>
>> A much better way to handle the use case is to wrap objects in a class
>> that gives this "propagating None" behavior with plain attribute access. A
>> nice implementation was presented in this thread.
>>
>>
>> On Sep 10, 2016 3:16 PM, "Random832" <random832 at fastmail.com> wrote:
>>>
>>> On Sat, Sep 10, 2016, at 13:26, Guido van Rossum wrote:
>>> > The way I recall it, we arrived at the perfect syntax (using ?) and
>>> > semantics. The issue was purely strong hesitation about whether
>>> > sprinkling ? all over your code is too ugly for Python
>>>
>>> I think that if there's "strong hesitation" about something being "too
>>> ugly" it can't really be described as "the perfect syntax". IIRC there
>>> were a couple alternatives being discussed that would have reduced the
>>> number of question marks to one [or one per object which might be None].
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>
>
>
>
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-- 
--Guido van Rossum (python.org/~guido)