Include/methodobject.h — Method Object Header

CPython exposes built-in functions to Python code as PyCFunctionObject instances. This header defines the typedefs, the PyMethodDef descriptor struct that every C extension fills in, and the full set of METH_* calling-convention flags introduced through 3.14.

Map

Lines	Symbol	Kind	Notes
1-10	PyCFunction	typedef	Single-argument C function pointer (PyObject*, PyObject*)
11-15	PyCFunctionWithKeywords	typedef	Three-argument variant adding a keyword dict
16-22	_PyCFunctionFast	typedef	Fastcall signature (PyObject*, PyObject* const*, Py_ssize_t)
23-30	_PyCFunctionFastWithKeywords	typedef	Fastcall plus keyword-names tuple
31-55	PyMethodDef	struct	ml_name, ml_meth, ml_flags, ml_doc
56-85	METH_* flags	defines	Calling-convention and binding flags
86-110	PyCFunctionObject	struct	Instance layout including vectorcall slot
111-130	PyMethodObject	struct	Bound-method wrapper
131-150	API functions	declarations	PyCFunction_New, PyCFunction_NewEx, PyCFunction_GET_* macros

Reading

PyCFunction typedefs

typedef PyObject *(*PyCFunction)(PyObject *self, PyObject *args);
typedef PyObject *(*PyCFunctionWithKeywords)(PyObject *self,
                                             PyObject *args,
                                             PyObject *kwargs);
typedef PyObject *(*_PyCFunctionFast)(PyObject *self,
                                      PyObject *const *args,
                                      Py_ssize_t nargs);
typedef PyObject *(*_PyCFunctionFastWithKeywords)(PyObject *self,
                                                   PyObject *const *args,
                                                   Py_ssize_t nargs,
                                                   PyObject *kwnames);

The plain PyCFunction pointer is the historic two-argument form. The _PyCFunctionFast family avoids building an argument tuple, passing a C array and a count instead. Callers select the right typedef by reading ml_flags.

PyMethodDef struct

struct PyMethodDef {
    const char  *ml_name;   /* method name */
    PyCMethodDef ml_meth;   /* implementation */
    int          ml_flags;  /* flags bits (METH_*) */
    const char  *ml_doc;    /* docstring or NULL */
};

A sentinel entry with ml_name == NULL terminates method tables. Extensions that define module or type methods embed a static array of these structs and pass it to PyModule_Create or PyType_Ready.

METH_* flags and the 3.14 FASTCALL+KEYWORDS combination

#define METH_VARARGS    0x0001
#define METH_KEYWORDS   0x0002
#define METH_NOARGS     0x0004
#define METH_O          0x0008
#define METH_CLASS      0x0010
#define METH_STATIC     0x0020
#define METH_FASTCALL   0x0080

METH_FASTCALL | METH_KEYWORDS selects _PyCFunctionFastWithKeywords. CPython 3.14 formalised this combination as a stable ABI entry point (previously it was considered internal). Binding flags METH_CLASS and METH_STATIC are mutually exclusive with each other and orthogonal to the calling-convention bits.

gopy notes

• PyMethodDef maps to the MethodDef struct in objects/function.go.
• gopy uses a Go function pointer (FuncGo) in place of a raw PyCFunction; the trampoline in vm/eval_call.go dispatches on the flags field using the same bitmask values.
• METH_FASTCALL | METH_KEYWORDS dispatch was added in the v0.12 tier-2 work; earlier calls fell through to the METH_VARARGS path.
• The vectorcall slot (vectorcallfunc) in PyCFunctionObject corresponds to VectorCall in objects/function.go; it is populated during type initialisation, not per-instance.