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

[Steven D'Aprano]

On Mon, Nov 06, 2017 at 11:33:10AM -0800, Barry Warsaw wrote:

> If we did make the change, it’s possible we would need a way to 
> explicit say that order is not preserved.  That seems a little weird 
> to me, but I suppose it could be useful. 

Useful for what?

Given that we will hypothetically have order-preserving dicts that 
perform no worse than unordered dicts, I'm struggling to think of a 
reason (apart from performance) why somebody would intentionally use a 
non-ordered dict. If performance was an issue, sure, it makes sense to 
have a non-ordered dict for when you don't want to pay the cost of 
keeping insertion order. But performance seems to be a non-issue.

I can see people wanting a SortedDict which automatically sorts the keys 
into some specified order. If I really work at it, I can imagine that 
there might even be a use-case for randomizing the key order (like 
calling random.shuffle on the keys). But if you are willing to use a 
dict with arbitrary order, that means that *you don't care* what order 
the keys are in. If you don't care, then insertion order should be no 
better or worse than any other implementation-defined arbitrary order.


> I like the idea previously 
> brought up that iteration order be deliberately randomized in that 
> case, but we’d still need a good way to spell that.

That would only be in the scenario that we decide *not* to guarantee 
insertion-order preserving semantics for dicts, in order to prevent 
users from relying on an implementation feature that isn't a language 
guarantee.


-- 
Steve