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

Steven D'Aprano steve at pearwood.info
Tue Nov 7 01:39:13 EST 2017

On Mon, Nov 06, 2017 at 10:17:23PM -0200, Joao S. O. Bueno wrote:

> And also, forgot along the discussion, is the big disadvantage that
> other Python implementations would have a quite
> significant overhead on mandatory ordered dicts.

I don't think that is correct. Nick already did a survey, and found that
C# (IronPython), Java (Jython and VOC) and Javascript (Batavia) all have
acceptable insertion-order preserving mappings. C++ (Nuitka) surely
won't have any problem with this (if C++ cannot implement an efficient
order-preserving map, there is something terribly wrong with the world).

As for other languages that somebody might choose to build Python on 
(the Parrot VM, Haskell, D, Rust, etc) surely we shouldn't be limiting 
what Python does for the sake of hypothetical implementations in 
"underpowered" languages?

I don't mean to imply that any of those examples are necessarily
underpowered, but if language Foo is incapable of supporting an
efficient ordered map, then language Foo is simply not good enough for a
serious Python implementation. We shouldn't allow Python's evolution to
be hamstrung by the requirement to support arbitrarily weak
implementation languages.

> One that was mentioned along the way is transpilers, with
> Brython as an example - but there might be others.

Since Brython transpiles to Javascript, couldn't it use the standard
Map object, which preserves insertion order?



    A Map object iterates its elements in insertion order

The EMCAScript 6 standard specifies that Map.prototype.forEach operates
over the key/value pairs in insertion order:



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