> If it turns out that there's a dict implementation that's faster by not > preserving order, collections.UnorderedDict could be added. > There could also be specialized implementations that pre-size the dict (cf: > C++ unordered_map::reserve), etc., etc. > But these are all future things, which might not be necessary.
I think that the problem with this is that for the most part, people will use
`dict`, and for most uses of `dict`, order doesn't matter (and it never has
before). Given that "arbitrary order" includes *any* fixed ordering (insertion
ordered, reverse insertion ordered, etc), the common case should keep the
existing "no order guarantee" specification. This gives interpreter authors
maximum freedom with a fundamental, widely used data type.
> Isn't ordered dict also useful for **kwargs?
If this is useful (and it seems like it would be), I think again a syntax
modification that allows users to indicate that they want a particular
implementation of **kwargs would be better than modifying dict semantics. It
could possibly be handled with a type-hinting like syntax:
def f(*args, **kwargs : OrderedKwargs):
Or a riff on the existing syntax:
def f(*args, ***kwargs):
def f(*args, ^^kwargs):
def f(*args, .**kwargs):
In this case, the only guarantee you'd need (which relatively minor compared to
a change in the dict semantics) would be that keyword argument order passed to
a function would be preserved as the order that it is passed into the `kwargs`
constructor. The old **kwargs syntax would give you a `dict` as normal, and the
new ^^kwargs would give you an OrderedDict or some other dict subclass with
guaranteed order.
On 11/05/2017 03:50 PM, Peter Ludemann via Python-Dev wrote:
>
>
> On 5 November 2017 at 12:44, Sven R. Kunze <srku...@mail.de> wrote:
>
>> +1 from me too.
>>
>> On 04.11.2017 21:55, Jim Baker wrote:
>>
>> +1, as Guido correctly recalls, this language guarantee will work well
>> with Jython when we get to the point of implementing 3.7+.
>>
>> On Sat, Nov 4, 2017 at 12:35 PM, Guido van Rossum <gu...@python.org>
>> wrote:
>>
>>> This sounds reasonable -- I think when we introduced this in 3.6 we were
>>> worried that other implementations (e.g. Jython) would have a problem with
>>> this, but AFAIK they've reported back that they can do this just fine. So
>>> let's just document this as a language guarantee.
>>>
>>> On Sat, Nov 4, 2017 at 10:30 AM, Stefan Krah <ste...@bytereef.org> wrote:
>>>
>>>>
>>>> Hello,
>>>>
>>>> would it be possible to guarantee that dict literals are ordered in v3.7?
>>>>
>>>>
>>>> The issue is well-known and the workarounds are tedious, example:
>>>>
>>>> https://mail.python.org/pipermail/python-ideas/2015-Decembe
>>>> r/037423.html
>>>>
>>>>
>>>> If the feature is guaranteed now, people can rely on it around v3.9.
>>>>
>>>>
>>>>
>>>> Stefan Krah
>>>>
>>>>
>>>>
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>>>
>>>
>>>
>>> --
>>> --Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
>>>
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