[Python-ideas] Dict literal use for custom dict classes

[Nathaniel Smith]

On Wed, Dec 16, 2015 at 10:47 AM, Mike Miller <python-ideas at mgmiller.net> wrote:
>
> On 2015-12-15 11:02, Paul Moore wrote:
>>
>> while it's a bit too special case on its own, one
>> possibility would be to have
>>
>>      callable{k1: v1, k2: v2, ...}
>>
>> be syntactic sugar for
>>
>>      callable([(k1, k1), (k2, v2), ...])
>
>
> Very interesting... I've faced the issue several times over the years when
> I've wanted to unpack values into a function call in an ordered manner (but
> it hasn't been available).  Perhaps::
>
>     callable{**odict}
>
> In fact with callables I'd even go so far as wish that ordered unpacking was
> the default somehow, though I guess that probably isn't possible due to
> history.

That's not so clear, actually! It turns out that PyPy was able to make
their regular 'dict' implementation ordered, while at the same time
making it faster and more memory-efficient compared to their previous
(CPython-like) implementation:

  http://morepypy.blogspot.com/2015/01/faster-more-memory-efficient-and-more.html

So in PyPy all these issues are automatically solved for free. The
$1e6-question these other proposals have to answer is, why not do what
PyPy did? Maybe there is a good reason not to, but it seems like it'll
be difficult to get consensus on moving forward on any of these other
more complicated proposals until someone has first made a serious
attempt at porting PyPy's dict to CPython and is able to clearly
describe why it didn't work.

(3.5 does have a faster C implementation of OrderedDict, thanks to
tireless efforts by Eric Snow -- https://bugs.python.org/issue16991 --
but this implementation uses a very different and less cache-efficient
strategy than PyPy.)

-n

-- 
Nathaniel J. Smith -- http://vorpus.org