[Python-Dev] Guarantee ordered dict literals in v3.7?

[Paul Ganssle]

> 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 <srkunze at 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 <guido at 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 <stefan at 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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