pycore_call.h

A compact internal header that collects every calling-convention adapter the interpreter uses. Public callers use PyObject_Call; this header is for the hot paths inside ceval.c, typeobject.c, and the import machinery.

Map

Lines	Symbol	Role
1–20	_PyObject_Call	Final dispatch to tp_call or vectorcall slot
21–38	_PyObject_FastCall	Positional-only vectorcall, no kwargs
39–52	_Py_CallFunctionObjArgs	Varargs tuple-style call
53–68	_PyObject_CallMethodIdOneArg	Deprecated _Py_Identifier fast single-arg method call
69–82	_PyObject_CallMethodIdObjArgs	Deprecated _Py_Identifier multi-arg method call
83–98	_PyStack_UnpackDict	Unpack a kwargs dict into a vectorcall kwnames array

Reading

Final dispatch: _PyObject_Call

PyObject_Call (the public API) validates arguments and then delegates to _PyObject_Call. The split lets internal callers skip the argument checks when they know the inputs are already valid.

// CPython: Include/internal/pycore_call.h:1 _PyObject_Call
extern PyObject *
_PyObject_Call(PyThreadState *tstate, PyObject *callable,
               PyObject *args, PyObject *kwargs);

The implementation prefers the vectorcall slot (Py_TPFLAGS_HAVE_VECTORCALL) over tp_call when both are present, converting the tuple args and dict kwargs into a flat stack array on the fly.

Positional-only fast path: _PyObject_FastCall

When a call site knows there are no keyword arguments it can skip building a kwnames tuple entirely.

// CPython: Include/internal/pycore_call.h:21 _PyObject_FastCall
extern PyObject *
_PyObject_FastCall(PyObject *func, PyObject *const *args, Py_ssize_t nargs);

This is the vectorcall ABI with kwnames = NULL. The eval loop uses this for CALL instructions where the compiler proved no **kwargs were passed.

Unpacking kwargs: _PyStack_UnpackDict

Vectorcall passes keywords as a kwnames tuple alongside the positional stack. When a callee only understands the old (args, kwargs) calling convention, _PyStack_UnpackDict converts back.

// CPython: Include/internal/pycore_call.h:83 _PyStack_UnpackDict
extern int
_PyStack_UnpackDict(PyThreadState *tstate,
                    PyObject *const *args, Py_ssize_t nargs,
                    PyObject *kwnames,
                    PyObject **p_stack, PyObject **p_kwdict);

The function allocates a new stack array for positional arguments and a fresh dict for keyword arguments, writing both through the out-parameters. The caller owns both and must decref the dict and free the stack with PyMem_Free.

Deprecated identifier callers

_Py_Identifier was the pre-3.9 intern-on-demand string mechanism. The two method-call helpers that use it are kept for compatibility with C extension modules that have not migrated to PyObject_CallMethodOneArg.

// CPython: Include/internal/pycore_call.h:53 _PyObject_CallMethodIdOneArg
extern PyObject *
_PyObject_CallMethodIdOneArg(PyObject *self, struct _Py_Identifier *name,
                             PyObject *arg);

// CPython: Include/internal/pycore_call.h:69 _PyObject_CallMethodIdObjArgs
extern PyObject *
_PyObject_CallMethodIdObjArgs(PyObject *obj, struct _Py_Identifier *name, ...);

Both functions resolve the identifier to an interned string, call _PyObject_GetMethod, and then dispatch through the vectorcall path. New code should use PyObject_CallMethodOneArg instead.

gopy notes

• gopy's call entry point is vm.Call in vm/eval_call.go. It mirrors the _PyObject_Call split: public callers go through objects.Call, which validates, then delegates to the internal callInternal that skips checks.
• _PyObject_FastCall maps to vm.FastCall in vm/eval_call.go. The vectorcall ABI is represented as func(args []*Object, kwnames *Tuple).
• _PyStack_UnpackDict is implemented in vm/eval_call.go as UnpackKwargs. The Go version returns ([]Object, map[string]Object, error) rather than using output parameters.
• _Py_Identifier has no counterpart in gopy. All internal string lookups use pre-interned *objects.Str values stored as package-level vars.

CPython 3.14 changes

• The vectorcall protocol (Py_TPFLAGS_HAVE_VECTORCALL) became the preferred path in 3.9. By 3.14 the _Py_Identifier-based helpers carry a deprecation notice and will be removed in 3.15.
• _PyObject_FastCall gained a tstate parameter in 3.12 (previously it used _PyThreadState_GET()). All internal callers were updated; the old two-argument form is gone in 3.14.
• _PyStack_UnpackDict is now declared _Py_NO_RETURN_ON_INTERNALERROR in debug builds to catch callers that ignore the error return.