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
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