[Import-SIG] On singleton modules, heap types, and subinterpreters

Stefan Behnel stefan_ml at behnel.de
Mon Jul 27 08:44:08 CEST 2015

Petr Viktorin schrieb am 26.07.2015 um 12:39:
> This is a follow-up to PEP 489 and discussions regarding per-module
> data PyState_FindModule.
> It turned out to be quite the rabbit hole. Apologies for the long
> mail, I hope it ends up sufficiently clear.
> Using single-phase initialization (the pre-PEP 489 solution),
> extension modules are effectively singletons – there's up to one
> instance of a particular module in any given subinterpreter. Cython
> modules only allow one instance *per process*.
> Using the new multiple-phase init, one can create several modules from
> one PyModuleDef – either (again) one per subinterpreter, or for
> testing purposes. As per the goal of PEP 489, this brings extension
> modules closer to how Python modules behave.
> The problem is that classes defined in a module don't have a reference
> to the module object.
> [lots of of other tricky details stripped]

Sorry for cutting it short here, but isn't this a hint that linking the new
initialisation to subinterpreter support might not be a good idea? I mean,
there are a couple of advantages of the new initialisation scheme, e.g. for
relative imports etc., which are completely unrelated to subinterpreters.
And yet, the PEP suggests that supporting the new module setup scheme
should indicate that subinterpreters are supported as well. Given how
complex the support seems to be for any non-trivial module, linking the two
use cases might end up preventing us from getting any benefit at all out of
this for quite some time.


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