But this requires introspection.
On Fri, Sep 25, 2020 at 22:46 Ricky Teachey <ri...@teachey.org> wrote:
> On Fri, Sep 25, 2020 at 11:50 PM Steven D'Aprano <st...@pearwood.info>
> wrote:
>
>> TL;DR:
>>
>>
>>
>>
>>
>> 1. We have to pass a sentinel to the setitem dunder if there is no
>>
>>
>> positional index passed. What should that sentinel be?
>>
>
> Isn't there a problem with this starting assumption?
>
> Say that I have item dunder code of the signature that is common today,
> and I have no interest in providing direct support for kwd args in my item
> dunders. Say also, kwd unpacking is supported.
>
> If a person unpacks an empty dictionary (something that surely will happen
> occasionally), and a SENTIAL is provided, this poses a pretty good
> possibility to lead to unintended behavior in many cases:
>
> class C:
> def __getitem__(self, index):
> ...
>
> d = {}
> c = C()
> c[**d]
>
> The above will call:
>
> c.__getitem__(SENTINEL)
>
> Who knows what the effect of c[SENTINEL] will be for so much existing code
> out there? Bugs are surely to be created, aren't they? So a lot of people
> will have to make changes to existing code to handle this with a sentinel.
> This seems like a big ugly problem to me that needs to be avoided.
>
> There's a third option: just prohibit keyword-only subscripts. I think
>>
>>
>> that's harsh, throwing the baby out with the bathwater. I personally
>>
>>
>> have use-cases where I would use keyword-only subscripts so I would
>>
>>
>> prefer options 1 or 2.
>>
>
> Maybe there's a middle way option that could avoid the particular problem
> outlined above:
>
> # Option 4: do use a sentinel at all, user provides own (if desired)
>
> obj[spam=1, eggs=2]
> # => calls type(obj).__getitem__(USER_PROVIDED_SENTINEL, spam=1,
> eggs=2)
>
> del obj[spam=1, eggs=2]
> # => calls type(obj).__delitem__(USER_PROVIDED_SENTINEL, spam=1,
> eggs=2)
>
> obj[spam=1, eggs=2] = value
> # => calls type(obj).__setitem__(USER_PROVIDED_SENTINEL, value,
> spam=1, eggs=2)
>
> How do we write the dunder item to allow for this? Simple: just require
> that if it is desired to support kwd-only item setting, the writer of the
> code has to provide a default-- ANY default-- to the value argument in
> setitem (and this default will never be used).
>
> I suggest the ellipses object as the standard convention:
>
> MyPreferredSentinel = object()
>
> class C:
> def __setitem__(self, index=MyPreferredSentinel, value=..., **kwargs):
> ...
>
> c=C()
> c[x=1] = "foo"
> # calls => c.__setitem__(MyPreferredSentinel, "foo", x=1)
>
> This option provides an additional benefit: if I don't want to provide
> support for the kwd-only case, I don't have to handle it explicitly--
> errors occur without any effort on my part, just as they do today (although
> the error itself is different):
>
> class C:
> def __setitem__(self, index, value, **kwargs):
> ...
>
> c = C()
> c[x=1] = "foo" # yay, this produces an error with no effort
>
> Providing a default sentinel nullifies this possibility. In that case,
> have to handle a kwd-only indexing myself:
>
> class C:
> def __setitem__(self, index, value, **kwargs):
> if index is SENTINEL:
> handle_kwd_args_only()
>
> ---
> Ricky.
>
> "I've never met a Kentucky man who wasn't either thinking about going home
> or actually going home." - Happy Chandler
>
>
>
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> --
--Guido (mobile)
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