[Python-Dev] Changing PyModuleDef.m_reload to m_slots (original) (raw)

Petr Viktorin encukou at gmail.com
Fri Apr 17 15:52:56 CEST 2015

Hello,

PEP 489, Redesigning extension module loading [0], is currently discussed on import-sig, but one question calls for a wider audience.

As a background, the PyModuleDef structure [1] is currently:

 struct PyModuleDef{
   PyModuleDef_Base m_base;
   const char* m_name;
   const char* m_doc;
   Py_ssize_t m_size;
   PyMethodDef *m_methods;
   inquiry m_reload;
   traverseproc m_traverse;
   inquiry m_clear;
   freefunc m_free;
 };

... where the m_reload pointer is unused, and must be NULL. My proposal is to repurpose this pointer to hold an array of slots, in the style of PEP 384's PyType_Spec [2], which would allow adding extensions -- both those needed for PEP 489 and future ones.

The result would be:

 typedef struct {
     int slot;
     void *value;
 } PyModuleDesc_Slot;

 typedef struct PyModuleDef {
     PyModuleDef_Base m_base;
     const char* m_name;
     const char* m_doc;
     Py_ssize_t m_size;
     PyMethodDef *m_methods;
     PyModuleDesc_Slot* m_slots;  /* <-- change here */
     traverseproc m_traverse;
     inquiry m_clear;
     freefunc m_free;
 } PyModuleDef;

The alternative is defining another struct for module definitions, with a parallel versions of functions to operate on it, and duplication on the lower level (e.g. a variant of PyModule_GetDef, variants for PyState_FindModule &c with the supporting registry, extra storage in module objects, two places to look in for GC/deallocation hooks).

The downside is that, strictly speaking, this would break API/ABI, though in pedantic details.

The field name could conceivably be used to get the NULL or in C99-style designated initializers, but since NULL is the only valid value, I don't see a reason for that. It could also be used for zeroing the structure after allocating a PyModuleDef dynamically.

ABI-wise, this changes a function pointer to a data pointer. To my knowledge, Python doesn't support a platform where this would matter, but I admit my knowledge is not complete.

Please share any concerns you might have about this change.

[0] https://www.python.org/dev/peps/pep-0489/ [1] https://docs.python.org/3/c-api/module.html#c.PyModuleDef [2] https://www.python.org/dev/peps/pep-0384/#type-objects

More information about the Python-Dev mailing list