08.11.17 04:33, Nick Coghlan пише:
On 8 November 2017 at 11:44, Nick Coghlan <ncogh...@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.
It may be easy and more efficient to break the order at insertion.
1. Insert in the reversed order.
2. Add at the end or at the begin, changing the order on every insertion.
2. Swap with an arbitrary item.
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