On Thu, Jul 29, 2021 at 4:52 PM Nick Coghlan <ncoghlan@gmail.com> wrote:

On Fri, 30 Jul 2021, 6:05 am Mark Shannon, <mark@hotpy.org> wrote:

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Hi Nick,

Our discussion on PEP 558 got me thinking "What is the simplest thing that would work?".

This is what I came up (in the form of a draft PEP): https://github.com/markshannon/peps/blob/pep-locals/pep-06xx.rst

It doesn't have O(1) len(f_locals), and it does break `PyEval_GetLocals()` but I think the that is a small price to pay for simplicity and consistency.

I don't think it is OK to break PyEval_GetLocals() when we really don't need to, and the proposal also discards all the feedback that I received on earlier iterations of PEP 558. (I particularly recommend reading Nathaniel's analysis of why returning the proxy from locals() would be more likely to cause bugs in existing code than it would be to eliminate any).

Heh, I was actually just re-reading PEP 558 and going to ask you to include more details to justify the complexity, as compared to something like Mark's latest proposal here -- I'd totally forgotten I wrote that old post :-). So that was a timely reminder! Looking at the references in the PEP, is this the writeup you're talking about? https://mail.python.org/pipermail/python-dev/2019-May/157738.html The conclusion there is:

I'm leaning towards saying that on net, [snapshot] beats [PEP-minus-tracing]: it's dramatically simpler, and the backwards incompatibilities that we've found so far seem pretty minor, on par with what we do in every point release. (In fact, in 3/4 of the cases I looked at, [snapshot] is actually what users seemed to trying to use in the first place.)

For [proxy] versus [snapshot], a lot depends on what we think of changing the semantics of exec(). [proxy] is definitely more consistent and elegant, and if we could go back in time I think it's what we'd have done from the start. Its compatibility is maybe a bit worse than [snapshot] on non-exec() cases, but this seems pretty minor overall (it often doesn't matter, and if it does just write dict(locals()) instead of locals(), like you would in non-function scope). But the change in exec() semantics is an actual language change, even though it may not affect much real code, so that's what stands out for me.

I *think* (please correct me if I'm wrong) that what that calls [PEP-minus-tracing] is now corresponds to the current PEP draft, and [proxy] corresponds to Mark's draft at the beginning of this thread? -n -- Nathaniel J. Smith -- https://vorpus.org

Bringing the public record up to date with a brief off-list discussion between Mark, Nathaniel and I: * Mark hasn't convinced me that getting rid of the frame value cache entirely for optimised frames is a good idea, so he's going to write that proposal up as a competing PEP. Once it has been drafted and is ready for review, he will request the SC assign a PEP delegate. * On the PEP 558 front, Mark's proposal has highlighted a few inefficiencies in my reference implementation, where the code still implicitly updates the frame value cache in cases where the cache being up to date may not matter to the proxy API client. So I'll be working on another iteration of the implementation that ensures each caching proxy instance (at worst) only pays the O(N) cache refresh price on the first less than O(N) operation that relies on the cache being up to date, rather than paying it every time "f_locals" is retrieved from the frame object. We still have plenty of time before 3.11b1, so we expect it will be a month or two before the two proposals are in a position to be compared directly. Cheers, Nick. On Fri, 30 Jul 2021, 5:25 pm Nick Coghlan, <ncoghlan@gmail.com> wrote:

On Fri, 30 Jul 2021, 2:30 pm Nathaniel Smith, <njs@pobox.com> wrote:

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For [proxy] versus [snapshot], a lot depends on what we think of

changing the semantics of exec(). [proxy] is definitely more consistent and elegant, and if we could go back in time I think it's what we'd have done from the start. Its compatibility is maybe a bit worse than [snapshot] on non-exec() cases, but this seems pretty minor overall (it often doesn't matter, and if it does just write dict(locals()) instead of locals(), like you would in non-function scope). But the change in exec() semantics is an actual language change, even though it may not affect much real code, so that's what stands out for me.

I *think* (please correct me if I'm wrong) that what that calls [PEP-minus-tracing] is now corresponds to the current PEP draft, and [proxy] corresponds to Mark's draft at the beginning of this thread?

At the locals() level, PEP 558 is now [snapshot] (due to your original analysis showing that it was strictly better than what I had at the time), while Mark's draft is indeed [proxy].

Cheers, Nick.

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-n

-- Nathaniel J. Smith -- https://vorpus.org