[Python-Dev] Python 3.6 dict becomes compact and gets a private version; and keywords become ordered

[Tim Delaney]

On 13 September 2016 at 10:28, Brett Cannon <brett at python.org> wrote:

>
>> I'd like to add one more documented constraint - that dict literals
>> maintain definition order (so long as the dict is not further modified).
>> This allows defining a dict literal and then passing it as **kwargs.
>>
>
> That would require all dictionaries keep their insertion order which we
> are explicitly not doing (at least yet). If you look at the PEPs that are
> asking for definition order they specify an "ordered mapping", not a dict.
> Making dict literals do this means dict literals become "order mapping
> literals" which isn't what they are; they are dict literals. I don't think
> we should extend this guarantee to literals any more than any other
> dictionary.
>

I'm not sure I agree with you, but I'm not going to argue too strongly
either (it can always be revisited later). I will note that a conforming
implementation could be that the result of evaluating a dict literal is a
frozen ordered dict which transparently changes to be a mutable dict as
required. There could well be performance and/or memory benefits from such
a dict implementation.

Personally I expect all Python 3.6 implementations will have
order-preserving dict as that's the easiest way to achieve the existing
guarantees. And that enough code will come to depend on an order-preserving
dict that eventually the decision will be made to retrospectively guarantee
the semantics.

Tim Delaney
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