[Python-Dev] Usefulness of binary compatibility accross Python versions? (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Sat Dec 16 13:37:54 EST 2017

On Sat, 16 Dec 2017 09:34:02 -0800 Guido van Rossum <guido at python.org> wrote:

I think it's more acceptable to require matching versions now than it was 10 years ago -- people are much more likely to use installer tools like pip and conda that can check version compatibility.

I think I'd be okay with dropping the flag-based mechanism you describe if we were to introduce a clear mechanism that always rejected a dynamically loaded module if it was compiled for a different Python version. This should happen without any cooperation from the module. Perhaps in Python.h we can introduce a reference to a variable whose name varies by version (major.minor, I think) and which is defined only by the interpreter itself. Or perhaps the version should be based on a separate ABI version.

Interestingly, Python 2 had such an API version check (though it would only emit a warning), it was removed as part of PEP 3121: https://github.com/python/cpython/commit/1a21451b1d73b65af949193208372e86bf308411#diff-4664e4ea04dc636b18070ba01cf42d06L39

I haven't been able to find the pre-approval discussion around PEP 3121, so I'm not sure why the API check was removed. The PEP (quite short!) also says nothing about it.

Currently, you can pass a module_api_version to PyModule_Create2(), but that function is for specialists only :-)

("""Most uses of this function should be using PyModule_Create() instead; only use this if you are sure you need it.""")

And the new multi-phase initialization API doesn't seem to support passing an API version: https://docs.python.org/3/c-api/module.html#multi-phase-initialization

Fortunately, nowadays all major platforms (Windows, Linux, macOS) tag C extension filenames with the interpreter version (*), e.g.:

So in practice there should be little potential for confusion, except when renaming the extension file?

(*) references: https://bugs.python.org/issue22980 https://docs.python.org/3.7/whatsnew/3.5.html#build-and-c-api-changes

Regards

Antoine.

More information about the Python-Dev mailing list