Include/descrobject.h — Descriptor Object Header

Descriptors are the backbone of Python's attribute lookup protocol. This header declares the two struct types that C extensions use to expose attributes (PyMemberDef for raw struct-field access and PyGetSetDef for get/set callbacks), the integer constants that name each member type, and the factory functions that build live descriptor objects from these definitions.

Map

Lines	Symbol	Kind	Notes
1-20	PyMemberDef	struct	name, type, offset, flags, doc
21-50	T_* constants	defines	Integer codes for member types (T_INT, T_FLOAT, etc.)
51-65	READONLY, READ_RESTRICTED, PY_AUDIT_READ	defines	Access-restriction flags
66-85	PyGetSetDef	struct	name, get, set, doc, closure
86-100	PyDescr_NewMethod etc.	declarations	Factory functions for each descriptor kind
101-120	PyDescr_IsData	declaration	Distinguishes data vs non-data descriptors

Reading

PyMemberDef struct

struct PyMemberDef {
    const char *name;   /* attribute name */
    int         type;   /* T_* type code */
    Py_ssize_t  offset; /* byte offset into the C struct */
    int         flags;  /* READONLY etc. */
    const char *doc;    /* docstring or NULL */
};

PyMember_GetOne and PyMember_SetOne use the type code plus offset to read or write a field directly inside the object's C memory. No Python-level getter or setter is involved, making this the fastest path for numeric slots.

T_* member type constants

#define T_SHORT       0
#define T_INT         1
#define T_LONG        2
#define T_FLOAT       3
#define T_DOUBLE      4
#define T_STRING      5
#define T_OBJECT      6
#define T_CHAR        7
#define T_BYTE        8
#define T_UBYTE       9
#define T_USHORT     10
#define T_UINT       11
#define T_ULONG      12
#define T_STRING_INPLACE 13
#define T_BOOL       14
#define T_OBJECT_EX  16
#define T_LONGLONG   17
#define T_ULONGLONG  18
#define T_PYSSIZET   19
#define T_NONE       20

T_OBJECT allows None when the field is NULL; T_OBJECT_EX raises AttributeError for NULL instead. T_NONE marks a slot that always returns None (used by sentinel entries).

PyGetSetDef struct and access flags

typedef PyObject *(*getter)(PyObject *obj, void *closure);
typedef int       (*setter)(PyObject *obj, PyObject *value, void *closure);

struct PyGetSetDef {
    const char *name;
    getter      get;
    setter      set;    /* NULL means read-only */
    const char *doc;
    void       *closure;
};

#define READONLY          1
#define READ_RESTRICTED   2
#define PY_AUDIT_READ     2   /* alias, triggers sys.audit */
#define WRITE_RESTRICTED  4
#define RESTRICTED        (READ_RESTRICTED | WRITE_RESTRICTED)

When set is NULL the descriptor behaves as read-only regardless of the flags field. PY_AUDIT_READ causes PyMember_GetOne to fire a sys.audit event before returning the value, used for sensitive fields like frame.f_locals.

gopy notes

• PyMemberDef and PyGetSetDef are represented as MemberDef and GetSetDef in objects/descr.go.
• gopy stores member offsets relative to the Go struct via unsafe.Offsetof; the T_* dispatch table lives in objects/descr.go as a switch statement rather than a function-pointer array.
• READONLY is enforced at the descriptor layer before the setter is reached, matching CPython's behaviour in Objects/descrobject.c.
• Data-descriptor priority (PyDescr_IsData returning true when set != NULL) is implemented in objects/type.go inside the MRO attribute lookup loop.