[Python-Dev] redesigning the extension module initialisation protocol (was: Strange artifacts with PEP 3121 and monkey-patching sys.modules (in csv, ElementTree and others))
eliben at gmail.com
Sun Aug 11 19:43:30 CEST 2013
On Sun, Aug 11, 2013 at 6:52 AM, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Nick Coghlan, 11.08.2013 15:19:
> > On 11 Aug 2013 09:02, "Stefan Behnel" wrote:
> >>> BTW, this already suggests a simple module initialisation interface.
> >>> extension module would expose a function that returns a module type,
> >>> the loader/importer would then simply instantiate that. Nothing else is
> >>> needed.
> >> Actually, strike the word "module type" and replace it with "type". Is
> >> there really a reason why Python needs a module type at all? I mean, you
> >> can stick arbitrary objects in sys.modules, so why not allow arbitrary
> >> types to be returned by the module creation function?
> > That's exactly what I have in mind, but the way extension module imports
> > currently work means we can't easily do it just yet. Fortunately,
> > means we now have some hope of fixing that :)
> Well, what do we need? We don't need to care about existing code, as long
> as the current scheme is only deprecated and not deleted. That won't happen
> before Py4 anyway. New code would simply export a different symbol when
> compiling for a CPython that supports it, which points to the function that
> returns the type.
> Then, there's already the PyType_Copy() function, which can be used to
> create a heap type from a statically defined type. So extension modules can
> simply define an (arbitrary) additional type in any way they see fit, copy
> it to the heap, and return it.
> Next, we need to define a signature for the type's __init__() method. This
> can be done in a future proof way by allowing arbitrary keyword arguments
> to be added, i.e. such a type must have a signature like
> def __init__(self, currently, used, pos, args, **kwargs)
> and simply ignore kwargs for now.
> Actually, we may get away with not passing all too many arguments here if
> we allow the importer to add stuff to the type's dict in between,
> specifically __file__, __path__ and friends, so that they are available
> before the type gets instantiated. Not sure if this is a good idea, but it
> would at least relieve the user from having to copy these things over from
> some kind of context or whatever we might want to pass in.
> Alternatively, we could split the instantiation up between tp_new() and
> tp_init(), and let the importer set stuff on the instance dict in between
> the two. But given that this context won't actually change once the shared
> library is loaded, the only reason to prefer modifying the instance instead
> of the type would be to avoid requiring a tp_dict for the type. Open for
> discussion, I guess.
> Did I forget anything? Sounds simple enough to me so far.
Out of curiosity - can we list actual use cases for this new design? The
previous thread, admittedly, deals with an isoteric corner-cases that comes
up in overly-clever tests. If we plan to serious consider these changes -
and this appears to be worth a PEP - we need a list of actual advantages
over the current approach. It's not that a more conceptually pure design is
an insufficient reason, IMHO, but it would be interesting to hear about
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