[Python-ideas] Unpacking a dict

[Davin Potts]

> So `a, b, c = **mapping` would be perfectly in line with this.

Your `func` example is a great connection to have made but I would not
reach the same conclusion.  When calling `func(**some_dict)` we are not
performing variable assignment but with the `a, b, c = **mapping` we would
be.  Whereas I accept that `func(**some_dict)` imposes some constraints on
the nature of the keys in the dict because functions must have proper
variable names as input parameters, I find it difficult to accept a similar
constraint on general dict unpacking.


Davin


On Thu, May 26, 2016 at 7:27 AM, Koos Zevenhoven <k7hoven at gmail.com> wrote:

> On Thu, May 26, 2016 at 5:18 AM, Guido van Rossum <guido at python.org>
> wrote:
> > On Wed, May 25, 2016 at 6:56 PM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> >> On Wed, May 25, 2016 at 01:11:35PM +0000, Michael Selik wrote:
> >> What is your evidence for this claim? So far I've only seen one real-
> >> world use-case for this, and that single use-case would be well served
> >> by a simpler syntax:
> >>
> >> a, b, c = **mapping
> >
> > I have to warn here. This looks cool but it does something that AFAIK
> > no other Python syntax uses -- it takes variable names and does
> > something to those variables but *also* uses their actual names as
> > string literals. I agree that the use cases for this seem pretty
> > sweet, but perhaps we should give it a somewhat different syntax just
> > so it's clear that the names on the LHS matter. The precedent that the
> > targets must be actual names rather than anything you can assign to is
> > also kind of scary.
>
> I understand the concern, and maybe you are right. However
> ​, this:​
>
> ​    ​
> def func(y, z, x)
> ​        ​
> print(x, y, z)
>
> ​    ​
> func(**dict(x=1, y=2, z=3))
>
> prints "1 2 3"
> ​, a
> ​nd so doe
> ​s​
> ​
>     func(x=1, y=2, z=3)​
>
> and
>
> ​    func(z=3, x=1, y=2)
>
> ​So `a, b, c = **mapping` would be perfectly in line with this. Of course
> there may still be confusion, but that would mean the user would probably
> already be confused about whether dicts are ordered or not, so that
> confusion would need to be fixed anyway. I think the key is that ** should
> _never_ be interpreted as unpack/repack by order. Or in other words, it
> always means unpack/repack _by name_.
>
> That said, here's a couple of suggestions:
>
>     **(a, b, c) = **mapping
>
>     **{a, b, c} = **mapping
>
> Although `a, b, c = **mapping` would still be more convenient.
>
> ​-- Koos
>
>
>
> PS. For even more explicitness:
>
> a from 'a', b from 'b', c from 'c' = **mapping
>
> Which would allow even
>
> b from 1, a from 0, x from 2 = **iterable
>
> Or
>
> a, b, c from 'a', 'b', 'c' in mapping
> b, a, c from 1, 0, 2 in mapping
>
>
> >
> > --
> > --Guido van Rossum (python.org/~guido)
> > _______________________________________________
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