[Python-Dev] Re: PEP 584 underspecified

[David Mertz]

I comment on the PR.

On Tue, May 19, 2020 at 4:44 PM Brandt Bucher 
wrote:

> Maybe I'm missing something here. The PEP specifically says:
>
> > Similarly, the iteration order of the key-value pairs in the dictionary
> will follow the same semantics as the examples above, with each newly added
> key (and its value) being appended to the current sequence.
>
> That seems sufficient, no?
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[Python-Dev] Re: PEP 584 underspecified

[Brandt Bucher]

Maybe I'm missing something here. The PEP specifically says:

> Similarly, the iteration order of the key-value pairs in the dictionary will 
> follow the same semantics as the examples above, with each newly added key 
> (and its value) being appended to the current sequence.

That seems sufficient, no?
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[Python-Dev] Re: PEP 584 underspecified

[Guido van Rossum]

Yes, the assumption (and the implementation) is that newly added keys are
inserted at the end, in the order in which the second object produces them
when iterated over. Please do send us a PR for the peps repo!

On Tue, May 19, 2020 at 12:34 PM David Mertz  wrote:

> I know this is late in the cycle to think of this.  But I just realized
> the PEP 584 does not say anything about order guarantees.
>
> I think it is safe to assume that existing keys in dictA will not have
> order modified by `dictA |= dictB`.  However, the PEP does not state what
> order we are given when new keys are added.  I reckon it is a kind of
> insertion, so they must occur after the existing keys.  However, stating
> explicitly that the MUST  (or alternately MIGHT NOT) be inserted in the
> order that dictB has seems worth specifying.
>
> Moreover, the new operator explicitly states that it will flexibly deal
> with mapping types other than dict on the right hand side.  So what happens
> with:
>
> dictA |= unorderedmapB
>
> I think the "reasonable" belief should be that all new keys will be in
> some unspecified order amongst themselves, but all occurring after the old
> keys.  I think a sentence or two explicitly indicating what is promised and
> what is not would be worthwhile.
>
>>
>>-
>>
>>PEP 584 , Union Operators
>>in dict
>>
>> --
> The dead increasingly dominate and strangle both the living and the
> not-yet born.  Vampiric capital and undead corporate persons abuse
> the lives and control the thoughts of homo faber. Ideas, once born,
> become abortifacients against new conceptions.
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