[Cython] Fwd: [Python-Dev] Re: Making code object APIs unstable

Tue Aug 17 19:37:39 EDT 2021

cython developers, heads up, please read the python-dev thread and respond
there.  (Most people on python-dev are unlikely to be able to communicate
with you via a cc to cython-devel@ due to the current
subscribers-only-posting mailing list config.)

-gps

---------- Forwarded message ---------
From: Gregory P. Smith <greg at krypto.org>
Date: Tue, Aug 17, 2021 at 4:24 PM
Subject: Re: [Python-Dev] Re: Making code object APIs unstable
To: Victor Stinner <vstinner at python.org>
Cc: Guido van Rossum <guido at python.org>, Nick Coghlan <ncoghlan at gmail.com>,
Python-Dev <python-dev at python.org>, <cython-devel at python.org>

+cc: cython-devel

background reading for those new to the thread:
https://mail.python.org/archives/list/python-dev@python.org/thread/ZWTBR5ESYR26BUIVMXOKPFRLGGYDJSFC/

On Tue, Aug 17, 2021 at 9:47 AM Victor Stinner <vstinner at python.org> wrote:

> Since Cython is a common consumer of this C API, can somone please dig
> into Cython to see exactly what it needs in terms of API? How does
> Cython create all arguments of the __Pyx_PyCode_New() macro? Does it
> copy an existing function to only override some fields, something like
> CodeType.replace(field=new_value)?
>
> If possible, I would prefer that Cython only uses the *public* C API.
> Otherwise, it will be very likely that Cython will break at every
> single Python release. Cython has a small team to maintain the code
> base, whereas CPython evolves much faster with a larger team.
>
> Victor
>

I don't claim knowledge of Cython internals, but the two places it appears
to call it's __Pyx_PyCode_New macro are:

https://github.com/cython/cython/blob/master/Cython/Utility/Exceptions.c#L769
in __Pyx_CreateCodeObjectForTraceback() - this one already has a `#if
CYTHON_COMPILING_IN_LIMITED_API` code path option in it.

 and

https://github.com/cython/cython/blob/master/Cython/Compiler/ExprNodes.py#L9722
in CodeObjectNode.generate_result_code()  that creates PyCodeObject's for
CyFunction instances per its comment.  Slightly described in this comment
http://google3/third_party/py/cython/files/Cython/Compiler/ExprNodes.py?l=397.
I don't see anything obvious mentioning the limited API in that code
generator.

it'd be best to loop in Cython maintainers for more of an idea of Cython's
intents and needs with PyCode_New APIs.  I've cc'd cython-devel at python.org.

-Greg

> On Tue, Aug 17, 2021 at 8:51 AM Gregory P. Smith <greg at krypto.org> wrote:
> >
> > Doing a search of a huge codebase (work), the predominant user of
> PyCode_New* APIs appears to be checked in Cython generated code (in all
> sorts of third_party OSS projects). It's in the boilerplate that Cython
> extensions make use of via it's __Pyx_PyCode_New macro.
> https://github.com/cython/cython/blob/master/Cython/Utility/ModuleSetupCode.c#L470
> >
> > I saw very few non-Cython uses.  There are some, but at a very quick
> first glance they appear simple - easy enough to reach out to the projects
> with a PR to update their code.
> >
> > The Cython use will require people to upgrade Cython and regenerate
> their code before they can use the Python version that changes these. That
> is not an uncommon thing for Cython. It's unfortunate that many projects on
> ship generated sources rather than use Cython at build time, but that isn't
> _our_ problem to solve. The more often we change internal APIs that things
> depend on, the more people will move their projects towards doing the right
> thing with regards to either not using said APIs or rerunning an up to date
> code generator as part of their build instead of checking in generated
> unstable API using sources.
> >
> > -gps
> >
> >
> > On Mon, Aug 16, 2021 at 8:04 PM Guido van Rossum <guido at python.org>
> wrote:
> >>
> >> On Mon, Aug 16, 2021 at 4:44 PM Nick Coghlan <ncoghlan at gmail.com>
> wrote:
> >>>
> >>> [...]
> >>> A cloning-with-replacement API that accepted the base code object and
> the "safe to modify" fields could be a good complement to the API
> deprecation proposal.
> >>
> >>
> >> Yes (I forgot to mention that).
> >>
> >>>
> >>> Moving actual "from scratch" code object creation behind the
> Py_BUILD_CORE guard with an underscore prefix on the name would also make
> sense, since it defines a key piece of the compiler/interpreter boundary.
> >>
> >>
> >> Yeah, we have _PyCode_New() for that.
> >>
> >>>
> >>> Cheers,
> >>> Nick.
> >>>
> >>> P.S. Noting an idea that won't work, in case anyone else reading the
> thread was thinking the same thing: a "PyType_FromSpec" style API won't
> help here, as the issue is that the compiler is now doing more work up
> front and recording that extra info in the code object for the interpreter
> to use. There is no way to synthesise that info if it isn't passed to the
> constructor, as it isn't intrinsically recorded in the opcode sequence.
> >>
> >>
> >> That's the API style that _PyCode_New() uses (thanks to Eric who IIRC
> pushed for this and implemented it). You gave me an idea now: the C
> equivalent to .replace() could use the same input structure; one can leave
> fields NULL that should be copied from the original unmodified.
> >>
> >> --
> >> --Guido van Rossum (python.org/~guido)
> >> Pronouns: he/him (why is my pronoun here?)
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>
>
> --
> Night gathers, and now my watch begins. It shall not end until my death.
>
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