[Python-Dev] What version is an extension module binary compatible with

[Martin Panter]

On 30 March 2017 at 15:31, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 29 March 2017 at 02:18, Paul Moore <p.f.moore at gmail.com> wrote:
>> On 28 March 2017 at 12:24, Miro Hrončok <mhroncok at redhat.com> wrote:
>>> I'd like some clarification on what ABI compatibility we can expect.
>>>  * Should the ABI be stable across patch releases (so calling
>>> PySlice_AdjustIndices from an existing macro would be a bug)?
>>>  * Should the ABI be forward-compatible within a minor release (so modules
>>> built for 3.6.0 should be usable with 3.6.1, but not vice versa)?
>>>  * Or should we expect the ABI to change even across patch releases?
>>
>> Given that binary wheels are built against a specific minor version
>> (3.6, 3.5, ...) I would expect the ABI to be consistent over a minor
>> release. That would fit with my expectations of the compatibility
>> guarantees on patch releases.
>>
>> So I from what you describe, I'd consider this as a bug. Certainly, if
>> someone built a C extension as a wheel using Python 3.6.1, it would be
>> tagged as compatible with cp36, and pip would happily use it when
>> installing to a Python 3.6.0 system, where it would fail.
>
> Right, this is the main problem - while "build against the X.Y.0
> headers" is useful advice, it's not something we've ever explicitly
> stated, and it's not something we can reasonably expect all providers
> of pre-built binary modules to do.
>
> Instead, it makes sense to explicitly strengthen the ABI guarantees
> within CPython maintenance releases, and add some automated testing to
> avoid accidental changes and oversights (similar to the pending test
> to ensure magic number stability for cached bytecode files)

There's a website that has nice reports of ABI compatibility of
various packages and might useful for testing. It shows up the added
PySlice_AdjustIndices function in 3.6.1, along with PySlice_Unpack
(and some changes to internal names, so probably not such a problem).
https://abi-laboratory.pro/tracker/compat_report/python/3.6.0/3.6.1/496a4/abi_compat_report.html