[Python-Dev] No longer enable Py_TRACE_REFS by default in debug build

Victor Stinner vstinner at redhat.com
Fri Apr 12 21:35:42 EDT 2019

>> The very first question I asked was whether this would let us converge
>> the ABIs, and the answer was "no".

The answer is yes and it's my primary goal.

See my first email: "This change makes the debug build ABI closer to the
release build ABI".

To be honest, I am now lost in this long thread :-) I don't recall why I
started to argue so much about the memory footprint, it's not really the
main point here.


>> Otherwise I'd have said go for it, despite the C runtime issues.
> I don't see that in the thread... just Victor saying he isn't sure
> whether there might be other ABI incompatibilities lurking that he
> hasn't found yet. Did I miss something?
> I'm mostly interested in this because of the possibility of converging
> the ABIs. If you think that the C runtime thing isn't a blocker for
> that, then that's useful information. Though obviously we still need
> to figure out whether there are any other blockers :-).
>> >>>> The reason we ship debug Python binaries is because debug builds
use a
>> >>>> different C Runtime, so if you do a debug build of an extension
>> >>>> you're working on it won't actually work with a non-debug build of
>> >>>
>> >>> ...But this is an important point. I'd forgotten that MSVC has a
>> >>> of changing the entire C runtime when you turn on the compiler's
>> >>> debugging mode.
>> >>
>> >> Technically they are separate options, but most project files are
>> >> configured such that *their* Debug/Release switch affects both the
>> >> compiler options (optimization) and the linker options (C runtime
>> >
>> > So how do other projects handle this? I guess historically the main
>> > target audience for Visual Studio was folks building monolithic apps,
>> > where you can just rebuild everything with whatever options you want,
>> > and compared to that Python extensions are messier. But Python isn't
>> > the only project in this boat. Do ruby, nodejs, R, etc., all provide
>> > separate debug builds with incompatible ABIs on Windows, and propagate
>> > that information throughout their module/package ecosystem?
>> Mostly I hear complaints about those languages *not* providing any help
>> here. Python is renowned for having significantly better Windows support
>> than any of them, so they're the wrong comparison to make in my opinion.
>> Arguing that we should regress because other languages haven't caught up
>> to us yet makes no sense.
>> The tools that are better than Python typically don't ship debug builds
>> either, unless you specifically request them. But they also don't leak
>> their implementation details all over the place. If we had a better C
>> API, we wouldn't have users who needed to match ABIs.
> Do you happen to have a list of places where the C API leaks details
> of the underlying CRT?
> (I'm mostly curious because whenever I've looked my conclusion was
> essentially: "Well....... I don't see any places that are *definitely*
> broken, so maybe mixing CRTs is fine? but I have zero confidence that
> I caught everything, so probably better to play it safe?". At least on
> py3 – I know the py2 C API was definitely broken if you mixed CRTs,
> because of the exposed FILE*.)
>> For the most part, disabling optimizations in your own extension but
>> using the non-debug ABI is sufficient, and if you're having to deal with
>> other people's packages then maybe you don't have any choice (though I
>> do know of people who have built debug versions of numpy before - turns
>> out Windows developers are often just as capable as non-Windows
>> developers when it comes to building things ;)
> I'm not sure why you think I was implying otherwise? I'm sorry if you
> thought I was attacking your users or something. I did say that I
> thought most users downloading the debug builds were probably confused
> about what they were actually getting, but I didn't mean because they
> were stupid Windows users, I meant because the debug builds are so
> confusing that even folks on the Python core team are confused about
> what they're actually getting.
> -n
> --
> Nathaniel J. Smith -- https://vorpus.org
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Night gathers, and now my watch begins. It shall not end until my death.
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