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

Ivan Levkivskyi levkivskyi at gmail.com
Fri Mar 1 10:32:03 EST 2019

On Fri, 1 Mar 2019 at 14:50, Eric V. Smith <eric at trueblade.com> wrote:

> 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.

OK, thanks for explaining! So more formally speaking, you want to say that
for other examples of '+' in Python
x1 + y == x2 + y if and only if x1 == x2, while for the proposed '+' for
dicts there may be many different x_i such that
x_i + y gives the same result.

This doesn't bother me either, since this is not a critical requirement for
addition. I would say this is rather a coincidence
than a conscious decision.

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