[Python-ideas] Dict joining using + and +=

[Eric V. Smith]

On 3/1/2019 9:38 AM, INADA Naoki wrote:
> Sorry, I'm not good at English enough to explain my mental model.
> 
> I meant no skip, no ignorance, no throw away.
> 
> In case of 1+2=3, both of 1 and 2 are not skipped, ignored or thrown away.
> 
> On the other hand, in case of {a:1, b:2}+{a:2}={a:2, b:2}, I feel {a:1} 
> is skipped, ignored, or thrown away.  I used "lost" to explain it.
> 
> And I used "lossless" for "there is no lost".  Not for reversible.
> 
> If it isn't understandable to you, please ignore me.
> 
> I think Rémi’s comment is very similar to my thought.  Merging mapping 
> is more complex than concatenate sequence and it seems hard to call it 
> "sum".

I understand Inada to be saying that each value on the LHS (as shown 
above) affects the result on the RHS. That's the case with addition of 
ints and other types, but not so with the proposed dict addition. As he 
says, the {a:1} doesn't affect the result. The result would be the same 
if this key wasn't present in the first dict, or if the key had a 
different value.

This doesn't bother me, personally. I'm just trying to clarify.

Eric

> 
> Regards,
> 
> 
> 2019年3月1日(金) 23:19 Ivan Levkivskyi <levkivskyi at gmail.com 
> <mailto:levkivskyi at gmail.com>>:
> 
>     On Fri, 1 Mar 2019 at 13:48, INADA Naoki <songofacandy at gmail.com
>     <mailto:songofacandy at gmail.com>> wrote:
> 
>          >
>          >
>          > Is that an invariant you expect to apply to other uses of the +
>          > operator?
>          >
>          > py> x = -1
>          > py> x <= (x + x)
>          > False
>          >
>          > py> [999] <= ([1, 2, 3] + [999])
>          > False
>          >
> 
>         Please calm down.  I meant each type implements "sum"
>         in semantics of the type, in lossless way.
>         What "lossless" means is changed by the semantics of the type.
> 
>         -1 + -1 = -2 is sum in numerical semantics.  There are no loss.
> 
> 
>     TBH I don't understand what is lossless about numeric addition. What
>     is the definition of lossless?
>     Clearly some information is lost, since you can't uniquely restore
>     two numbers you add from the result.
> 
>     Unless you define what lossless means, there will be just more
>     misunderstandings.
> 
>     --
>     Ivan
> 
> 
> 
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