On 17 July 2018 at 16:59, Cosimo Lupo firstname.lastname@example.org wrote:
I would like to revive this 5 year old thread and see if we can stir things up a bit.
Basically the problem is that, in the current state of the PEPs and build tools, it is still not possible to build and distribute a Windows wheel that includes an extension module compiled with Py_LIMITED_API. Setuptools can successfully build such extension module on Windows (with `.pyd` file extension and no extra specifiers in the module filename), and these seems to work at least on CPython 3.5 and above. However the `--py-limited-api cpXX` option of bdist_wheel command fails on Windows because it attempts to use the `abi3` tag but the latter is not in the list of compatible tags for that platform. One can work around this by creating a wheel with `none` as the abi tag, and `cp35.cp36.cp37` as the python implementation tag but this feels a bit hackish.
Here are some unresolved questions from the old distutils-sig thread, quoting Paul Moore:
- How should tools determine which ABIs a given Python supports?
(This is the get_supported function in wheel and distlib). The "Use" section of the PEP (http://www.python.org/dev/peps/pep-0425/#id1) gives a Linux-based example, but nothing normative and nothing that is understandable to a Windows user.
And from Vinay Sajip
For Windows, we (eventually) need some low-level sysconfig-supported way to get the ABI information in an analogous way to how it happens on POSIX: and because that's not currently there, distlib doesn't provide any ABI information on Windows other than "none".
So.. what needs to be done here to allow distributing/installing Windows wheels with Py_LIMITED_API support?
IMO, the question I posed back then remains key. Vinay's response is fair, but I don't think that waiting for core Python to provide something via sysconfig is practical (it's not happened yet, so why would we expect things to change?). So I think the next step is probably for someone to propose an algorithm that can be used. Actually, what I'd like to see is a full end to end proposal of the process someone would use to build and install a limited-ABI extension. That would probably tease out a number of issues.
I imagine the steps would be something like this:
1. Create an extension. Presumably you'd need to #define PY_LIMITED_ABI in the source. 2. Build a wheel from it - how would you do that? I gather it's possible to do this with plain setuptools - would it be necessary to do this with setuptools/bdist_wheel, or should there be a way to request a limited ABI build via pip? If we do want to be able to request this from a build tools like pip, do we need something in PEP 517? Are we only looking at the prebuilt wheel case, or do we need to support building from source? 3. What tags would the built wheel have? 4. Install that wheel - `pip install xxx`. Pip needs to be able to enumerate the full list of valid tags here (cp37-abi3, cp3-abi3, ...) There are also questions like - if there's a limited ABI wheel and a full ABI (version specific) wheel, which takes precedence?
I don't honestly know how well the limited ABI actually achieves its goals - is "cp3-abi3-win_x86_64" a realistic tag to apply? Can limited ABI wheels really be used on any version of Python 3? That's a question for python-dev, rather than distutils-sig, but if we take the position that this is what's promised, and it later turns out not to be true, we've got a lot of wheel renaming to do when Python 3.10 comes out and it doesn't support the same limited ABI as 3.x for x < 10...
Also, does the limited ABI work on other platforms? If it does, we should ensure that the Windows support works the same. If it doesn't, do we want a Windows-only solution (why is that OK?) or should we extend to (say) manylinux or OSX (at the risk of making the job too big for anyone to actually get anywhere with it).
So to move this forward, I think someone needs to write up the process of building and using a limited ABI wheel, as described above, and document suggested answers to the various questions that will come out in the process of going through the details. Is that something you'd be willing to take on?
From that, we'd have something concrete to debate. I'm not sure how many people have an interest in this topic, so getting people with the necessary knowledge to chime in might take some work (I'm interested, but I don't have detail understanding of build options and linking conventions). The ultimate goal would be some sort of PEP covering handling limited ABI extensions within the packaging infrastructure.
Does that seem reasonable? Is that the sort of guidance you were looking for? I doubt anything is going to happen on this subject until someone with the interest in moving it forward steps up to do the work of making a proposal and collecting community views.