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

[Eric Snow]

On Wed, Jul 29, 2015 at 2:01 PM, Petr Viktorin <encukou at gmail.com> wrote:
> On Wed, Jul 29, 2015 at 8:57 PM, Eric Snow <ericsnowcurrently at gmail.com> wrote:
>> The slot methods would have to do `type(self).<"global">`, which is
>> what any other method would do.  So in the relevant _csv.reader
>> methods we would use the equivalent "type(self).Error" where needed.
>
> We need the class that defines the method. type(self) might return a
> subclass of that. So we either need to walk the MRO until the defining
> class is found, or use Nick's mechanism and record the defining class
> for each relevant special method.

If you explicitly bind the module-scoped object to the class that
needs it, then the methods of that class can access it.  There's no
need for anything more complicated (even for type slot methods).  In
Python it would look like this:

    class Error(Exception):
        ...

    class Spam:
        Error = Error
        def fail(self):
            # Look up Error on Spam instead of from globals().
            raise type(self).Error()

We could even get more generic about it:

    class Spam:
        __globals__ = globals()
        def fail(self):
            raise self.__globals__["Error"]()

Obviously it's not that simple if we are trying to provide an implicit
Python-style scoping lookup for C extension functions/methods...

>> However, it sounds like you're suggesting that PyState_FindModule
>> should be fixed, replaced, or supplemented, assuming I've understood
>> correctly that it's part of the problem here.  I'd say it depends on
>> the actual impact of the lack of implicit Python-level scoping/lookup
>> in C extension functions/methods.
>
> PyState_FindModule bypasses the need for passing a module reference
> around: you give it a PyModuleDef (static data from which a module is
> constructed), and from that it looks up the corresponding module in
> current subinterpreter state.
> This  assumes the same set of modules is loaded in every
> subinterpreter. (Or at least that the required module is loaded in the
> current subinterpreter.)
> If we want to support subinterpreters safely, PyState_FindModule must go.

Hence we *are* looking for any alternative lookup mechanism
(effectively an equivalent to Python's scoping lookup).  So my
question is, is it worth it relative to C extension functions/methods?

-eric