Also worth noting is that dict.skip would live in dict.__dict__... which is a
dict.
Brandt
> On Sep 5, 2019, at 08:51, Andrew Barnert via Python-ideas
> <python-ideas@python.org> wrote:
>
>> On Sep 4, 2019, at 15:25, r...@hwyl.org wrote:
>>
>> ..."skip" being a unique object defined in and accessible from dict (there
>> may be better places to make it available). The dict insertion code then
>> simply ignores those entries when creating the dict.
>
> I’m a bit confused about exactly when this does and doesn’t come into play.
>
> You say here “the dict insertion code” does it. Would they mean d[key]=d.skip
> doesn’t insert anything? If so, then how could I put skip in a dict at all?
> If there’s already a d[key] does that get left alone, or deleted? Wouldn’t
> this even affect dict inserts that happen behind the scenes, like any time
> you tried to assign dict.skip to a global variable or a class or instance
> attribute, or pass it as a keyword argument? If an implementation doesn’t use
> dict.insert to build dict literals, does that mean it doesn’t work in dict
> literals on that implementation, even though that was the whole motivation
> for the feature?
>
> Maybe instead of starting with how it could be implemented, start with where
> you ideally do and don’t want it to take effect, and then we can try to work
> out how it could be implemented to do that, and how to specify it to require
> such an implementation.
>
> * Obviously you want it in non-comprehension dict displays, because that’s
> your example.
> * What about comprehensions?
> * Constructor calls with the keyword-arg API like dict(a=1, b=dict.skip,
> **d2}?
> * Constructor calls from iterables of pairs, like dict((1, 2), (3,
> dict.skip))?
> * Subscription assignments?
> * Calls to insert?
> * Calls to update?
> * Capturing ** keyword args?
> * Assignments to globals, attributes, etc. when those use a dict under the
> covers (as they usually do!?
> * Assignments to such things when they don’t use a dict (as with a slots
> class instance)?
>
> Also, what should happen with this case:
>
> flag0, flag1 = True, False
> d = {'k': 0 if flag0 else dict.skip,
> 'k': 1 if flag1 else dict.skip}
>
> Do we keep the last value for 'k' as usual, and because that’s dict.skip,
> therefore d['k'] ends up as a KeyError? Or do we keep the last non-skip value
> and therefore d['k'] ends up as 0?
>
> What about this:
>
> class D(dict):
> skip = 0
> d = D()
> d.skip = 1
> d[0] = 0
> d[1] = 1
> d[dict.skip] = dict.skip
>
> Do we skip the one that uses d.skip, the one that uses type(d).skip (like a
> special method lookup), or the one that uses dict.skip?
>
>
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