Objects/methodobject.c

cpython 3.14 @ ab2d84fe1023/Objects/methodobject.c ↗

Objects/methodobject.c defines PyCFunctionObject, the Python-visible wrapper around a C function pointer (PyCFunction). It also defines PyCMethodObject, a subtype that carries a PyObject *cls for classmethods. The file implements the call dispatch that translates Python calling conventions (*args, **kwargs) into the correct C ABI for METH_VARARGS, METH_FASTCALL, METH_NOARGS, and METH_O flags.

Map

Lines	Symbol	Role
1-60	PyCFunctionObject struct	ml, self, module, weakreflist fields
61-180	PyCFunction_NewEx, PyCMethod_New	Constructor paths
181-350	meth_call, meth_vectorcall	Call dispatch for all METH_* flags
351-500	richcompare, hash, repr	Type slots
501-620	PyCMethodObject, PyCMethod_Type	Classmethod subtype
621-730	PyCFunction_Type	Type object registration

Reading

PyCFunctionObject struct

// CPython: Objects/methodobject.c:12 PyCFunctionObject
typedef struct {
    PyObject_HEAD
    PyMethodDef *m_ml;      /* Description of the C function */
    PyObject    *m_self;    /* Passed as 'self' arg to the C func */
    PyObject    *m_module;  /* The __module__ attribute, can be anything */
    PyObject    *m_weakreflist; /* List of weak references */
    vectorcallfunc vectorcall;
} PyCFunctionObject;

m_ml points to a PyMethodDef which carries the function name, C pointer, METH_* flags, and docstring. vectorcall is set at construction time to the appropriate dispatcher based on m_ml->ml_flags.

meth_vectorcall: calling convention dispatch

The vectorcall entry point inspects ml_flags and routes to the correct C ABI. METH_NOARGS and METH_O are the fast paths; METH_FASTCALL passes the internal argument vector directly.

// CPython: Objects/methodobject.c:210 meth_vectorcall
static PyObject *
meth_vectorcall(PyObject *func, PyObject *const *stack,
                size_t nargsf, PyObject *kwnames)
{
    Py_ssize_t nargs = PyVectorcall_NARGS(nargsf);
    PyCFunctionObject *m = (PyCFunctionObject *)func;
    PyMethodDef *ml = m->m_ml;

    switch (ml->ml_flags & (METH_VARARGS | METH_FASTCALL | METH_NOARGS |
                            METH_O | METH_KEYWORDS | METH_METHOD)) {
    case METH_NOARGS:
        ...
    case METH_O:
        ...
    case METH_VARARGS | METH_KEYWORDS:
        ...
    }
}

PyCMethod_New: classmethod variant

PyCMethodObject extends PyCFunctionObject with a mm_class pointer. PyCMethod_New sets mm_class to the defining class so that super() inside C classmethods can resolve the correct MRO entry.

// CPython: Objects/methodobject.c:530 PyCMethod_New
PyObject *
PyCMethod_New(PyMethodDef *ml, PyObject *self, PyObject *module,
              PyTypeObject *cls)
{
    PyCMethodObject *op = PyObject_GC_New(PyCMethodObject, &PyCMethod_Type);
    ...
    op->mm_class = (PyObject *)cls;
    Py_INCREF(cls);
    return (PyObject *)op;
}

gopy notes

Not yet ported directly. In gopy, built-in functions are represented as Go functions registered via objects/function.go or as method slots on type objects. The METH_* flag dispatch maps to Go function signatures. PyCMethodObject is the foundation for super().__init_subclass__ calls, which will be needed when classmethods on user-defined types are fully supported.

CPython 3.14 changes

3.14 added the METH_COEXIST flag handling audit and improved the vectorcall fast path for METH_FASTCALL | METH_KEYWORDS to avoid a tuple allocation in the common case.