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

[INADA Naoki]

> 2. Switches keyword args and class body execution namespaces over to
> odict so the test suite passes again
> 3. Measures the impact such a change would have on the benchmark suite

For now, odict use twice memory and 2x slower on iteration.
https://bugs.python.org/issue31265#msg301942

INADA Naoki  <songofacandy at gmail.com>


On Wed, Nov 8, 2017 at 11:33 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 8 November 2017 at 11:44, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> 2. So far, I haven't actually come up with a perturbed iteration
>> implementation that doesn't segfault the interpreter. The dict
>> internals are nicely laid out to be iteration friendly, but they
>> really do assume that you're going to start at index zero, and then
>> iterate through to the end of the array. The bounds checking and
>> pointer validity testing becomes relatively fiddly if you try to push
>> against that and instead start iteration from a point partway through
>> the storage array.
>
> In case anyone else wants to experiment with a proof of concept:
> https://github.com/ncoghlan/cpython/commit/6a8a6fa32f0a9cd71d9078fbb2b5ea44d5c5c14d
>
> I think we've probably exhausted the utility of discussing this as a
> purely hypothetical change, and so the only way to move the discussion
> forward will be for someone to draft a patch that:
>
> 1. Perturbs iteration for regular dicts (it's OK for our purposes if
> it's still deterministic - it just shouldn't match insertion order the
> way odict does)
> 2. Switches keyword args and class body execution namespaces over to
> odict so the test suite passes again
> 3. Measures the impact such a change would have on the benchmark suite
>
> My experiment is a starting point, but it will still be a fair bit of
> work to get it from there to a viable proof of concept that can be
> assessed against the status quo.
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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