[Python-Dev] Update - Re: Make extension module initialisation more like Python module initialisation

[Nick Coghlan]

On Fri, Nov 9, 2012 at 12:32 AM, Stefan Behnel <stefan_ml at behnel.de> wrote:

> here's an updated proposal, adopting Marc-Andre's improvement that uses a
> new field in the PyModuleDef struct to register the callback. Note that
> this change no longer keeps up binary compatibility, which may or may not
> be acceptable for Python 3.4.
>

It's not acceptable, as PyModuleDef is part of PEP 384's stable ABI. All
such public structures are locked at their original size.

3) The original proposal used a new C-API function to register the callback
> explicitly, as opposed to extending the PyModuleDef struct. This has the
> advantage of keeping up binary compatibility with existing Py3.3
> extensions. It has the disadvantage of adding another indirection to the
> setup procedure where a static function pointer would suffice.
>

Module initialisation is (and must be) part of the stable ABI. Indirection
(especially through Python) is a *good* thing, as, ideally, any new
interfaces should be defined in a way that doesn't increase the maintenance
burden for the stable ABI.

I don't agree that the use of a new init API can fail silently, so long as
it completely *replaces* the old API, rather than being an addition. That
way, since you won't be defining the *old* init function at all, old
versions will correctly refuse to load your module.

So I propose that we simply *fix* extension module loading to work the same
way as everything else: the loader creates the module object, and passes it
in to a new init function to be fully populated. __file__ and __name__
would be passed in as preinitialised module attributes. The existing
PyModule_Create functions would be complemented by a PyModule_SetDef
function which allowed a PyModuleDef to be configured on a pre-existing
module.

Extension modules that wanted to create multiple Python modules would still
be free to do so - it would just be up to the extension initialisation code
to call PyModule_Create to construct them and set __file__ based on the
__file__ of the passed in module.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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