PEP 820 – PySlot: Unified slot system for the C API

PEPs
4

This seems like a worthwhile improvement. Count me as a +1

I have a question:

What do you anticipate _sl_reserved being used for, or is it really just padding?
If you do anticipate a use, could we give it a name that might make that usage easier in future?

And a comment:

The struct

typedef struct PySlot {
    uint16_t sl_id;
    uint16_t sl_flags;
    union {
        uint32_t _sl_reserved;  // must be 0
    };
    union {
        void *sl_ptr;
        void (*sl_func)(void);
        Py_ssize_t sl_size;
        int64_t sl_int64;
        uint64_t sl_uint64;
    };
} PySlot;

contains anonymous unions.
I thought we generally avoided anonymous unions in the API, as C++ doesn’t support them.
How about:

union _Py_slot_value {
      void *ptr;
      void (*func)(void);
      Py_ssize_t size;
      int64_t int64;
      uint64_t uint64;
};

typedef struct PySlot {
    uint16_t sl_id;
    uint16_t sl_flags;
    uint32_t _sl_reserved;  // must be 0
    union _Py_slot_value sl_val;
} PySlot;

So, instead of:

   {  
       .sl_id=Py_tp_getattro,
       .sl_flags=PySlot_HAS_FALLBACK,
       .sl_func=myClass_getattro,
   },

you’d have

   {  
       .sl_id=Py_tp_getattro,
       .sl_flags=PySlot_HAS_FALLBACK,
       .sl_val.func=myClass_getattro,
   },
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