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[Python-ideas] Module lifecycle: simple alternative to PEP 3121/PEP 489

Petr Viktorin encukou at gmail.com
Fri Apr 15 09:15:07 EDT 2016 
Let's move the discussion to import-sig, as Nick explained in the other
subthread. Please drop python-ideas from CC when you reply.

On 04/15/2016 12:58 PM, Nikita Nemkin wrote:
> On Fri, Apr 15, 2016 at 2:24 PM, Petr Viktorin <encukou at gmail.com> wrote:
>>>
>>> 1) define a new "calling convention" flag like METH_GLOBALS.
>>> 2) store module ref in PyCFunctionObject.m_module
>>>     (currently it stores only the module name)
>>
>> Wouldn't that break backwards compatibility, though?
> 
> It will, and I consider this level of breakage acceptable. Alternatively,
> another field can be added to this struct.
> 
>> My planned approach is a bit more flexible:
>> - Add a reference to the module (ht_module) to heap types
>> - Create a calling convention METH_METHOD, where methods are passed the
>> class that defines the method (which might PyTYPE(self) or a superclass
>> of it)
>>
>> This way methods can get both module state and the class they are
>> defined on, and the replacement for PyModule_State is two indirections.
> 
> I've read the linked import-sig thread and realized the depth of issues
> involved...
> Those poor heap type methods don't even have access their own type
> pointer! In the light of that, your variant is more useful than mine.
> 
> Still, without a good slot support option, new METH_X conventions
> don't look attractive at all. Such fundamental change, but only solves
> half of the problem.

Well, it solves the problem for methods that have calling conventions,
and I'm pretty sure by now that a full solution will need this *plus*
another solution for slots. So I'm looking at the problems as two
separate parts, and I also think that when it comes to writing PEPs,
having two separate PEPs would make this more understandable.

> Also, MRO walking is actually not as slow as it seems.
> Checking Py_TYPE(self) and Py_TYPE(self)->tp_base *inline*
> will minimize performance overhead in the (very) common case.

I think so as well. This would mean that module state access in named
methods is fast; in slot methods it's possible (and usually fast
*enough*), and the full solution with __typeslots__ would still be possible.

> If non-slot methods had a suitable static anchor (the equivalent
> of slot function address for slots), they could use MRO walking too.

I think a new METH_* calling style and explicit pointers is a better
alternative here.
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