[Python-Dev] Strange artifacts with PEP 3121 and monkey-patching sys.modules (in csv, ElementTree and others) (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sun Aug 11 15:19:41 CEST 2013

On 11 Aug 2013 09:02, "Stefan Behnel" <stefan_ml at behnel.de> wrote:

Stefan Behnel, 11.08.2013 14:53: > Stefan Behnel, 11.08.2013 14:48: >> Antoine Pitrou, 11.08.2013 14:32: >>> On Sun, 11 Aug 2013 14:16:10 +0200 Stefan Behnel wrote: >>>>> We >>>>> just need to devise a convenience API for that (perhaps by allowing to >>>>> create both the subclass and instantiate it in a single call). >>>> >>>> Right. This conflicts somewhat with the simplified module creation. If the >>>> module loader passed the readily instantiated module instance into the >>>> module init function, then module subtypes don't fit into this scheme anymore. >>>> >>>> One more reason why modules shouldn't be special. Essentially, we need an >>>> mnew() and minit() for them. And the lifetime of the module type would >>>> have to be linked to the (sub-)interpreter, whereas the lifetime of the >>>> module instance would be determined by whoever uses the module and/or >>>> decides to unload/reload it. >>> >>> It may be simpler if the only strong reference to the module type is in >>> the module instance itself. Successive module initializations would get >>> different types, but that shouldn't be a problem in practice. >> >> Agreed. Then the module instance would just be the only instance of a new >> type that gets created each time the module initialised. Even if module >> subtypes were to become common place once they are generally supported >> (because they're the easiest way to store per-module state efficiently), >> module reinitialisation should be rare enough to just buy them with a new >> type for each. The size of the complete module state+dict will almost >> always outweigh the size of the one additional type by factors. > > BTW, this already suggests a simple module initialisation interface. The > extension module would expose a function that returns a module type, and > 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, importlib means we now have some hope of fixing that :)

Cheers, Nick.

Stefan

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