bpo-37194: Add a new public PyObject_CallNoArgs() function (GH-13890)

Add a new public PyObject_CallNoArgs() function to the C API: call a
callable Python object without any arguments.

It is the most efficient way to call a callback without any argument.
On x86-64, for example, PyObject_CallFunctionObjArgs(func, NULL)
allocates 960 bytes on the stack per call, whereas
PyObject_CallNoArgs(func) only allocates 624 bytes per call.

It is excluded from stable ABI 3.8.

Replace private _PyObject_CallNoArg() with public
PyObject_CallNoArgs() in C extensions: _asyncio, _datetime,
_elementtree, _pickle, _tkinter and readline.

Doc/c-api/object.rst

@@ -253,6 +253,16 @@ Object Protocol
    and ``0`` otherwise.  This function always succeeds.
 
 
+.. c:function:: PyObject* PyObject_CallNoArgs(PyObject *callable)
+
+   Call a callable Python object *callable* without any arguments.
+
+   Returns the result of the call on success, or raise an exception and return
+   *NULL* on failure.
+
+   .. versionadded:: 3.9
+
+
 .. c:function:: PyObject* PyObject_Call(PyObject *callable, PyObject *args, PyObject *kwargs)
 
    Call a callable Python object *callable*, with arguments given by the

Doc/whatsnew/3.9.rst

@@ -102,6 +102,8 @@ Optimizations
 Build and C API Changes
 =======================
 
+* Add a new public :c:func:`PyObject_CallNoArgs` function to the C API:
+  call a callable Python object without any arguments.
 
 
 Deprecated

Include/abstract.h

@@ -141,6 +141,12 @@ extern "C" {
 #endif
 
 
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03090000
+/* Call a callable Python object without any arguments */
+PyAPI_FUNC(PyObject *) PyObject_CallNoArgs(PyObject *func);
+#endif
+
+
 /* Call a callable Python object 'callable' with arguments given by the
    tuple 'args' and keywords arguments given by the dictionary 'kwargs'.
 

Include/cpython/abstract.h

@@ -144,7 +144,9 @@ _PyObject_FastCall(PyObject *func, PyObject *const *args, Py_ssize_t nargs)
     return _PyObject_Vectorcall(func, args, (size_t)nargs, NULL);
 }
 
-/* Call a callable without any arguments */
+/* Call a callable without any arguments
+   Private static inline function variant of public function
+   PyObject_CallNoArgs(). */
 static inline PyObject *
 _PyObject_CallNoArg(PyObject *func) {
     return _PyObject_Vectorcall(func, NULL, 0, NULL);

Misc/NEWS.d/next/C API/2019-06-07-14-03-52.bpo-37194.uck7MD.rst

+Add a new public :c:func:`PyObject_CallNoArgs` function to the C API: call a
+callable Python object without any arguments. It is the most efficient way to
+call a callback without any argument. On x86-64, for example,
+``PyObject_CallFunctionObjArgs(func, NULL)`` allocates 960 bytes on the stack
+per call, whereas ``PyObject_CallNoArgs(func)`` only allocates 624 bytes per
+call.

Modules/_asynciomodule.c

@@ -216,7 +216,7 @@ get_future_loop(PyObject *fut)
         return NULL;
     }
     if (getloop != NULL) {
-        PyObject *res = _PyObject_CallNoArg(getloop);
+        PyObject *res = PyObject_CallNoArgs(getloop);
         Py_DECREF(getloop);
         return res;
     }
@@ -328,7 +328,7 @@ get_event_loop(void)
         return loop;
     }
 
-    policy = _PyObject_CallNoArg(asyncio_get_event_loop_policy);
+    policy = PyObject_CallNoArgs(asyncio_get_event_loop_policy);
     if (policy == NULL) {
         return NULL;
     }
@@ -510,7 +510,7 @@ future_init(FutureObj *fut, PyObject *loop)
            method, which is called during the interpreter shutdown and the
            traceback module is already unloaded.
         */
-        fut->fut_source_tb = _PyObject_CallNoArg(traceback_extract_stack);
+        fut->fut_source_tb = PyObject_CallNoArgs(traceback_extract_stack);
         if (fut->fut_source_tb == NULL) {
             return -1;
         }
@@ -553,7 +553,7 @@ future_set_exception(FutureObj *fut, PyObject *exc)
     }
 
     if (PyExceptionClass_Check(exc)) {
-        exc_val = _PyObject_CallNoArg(exc);
+        exc_val = PyObject_CallNoArgs(exc);
         if (exc_val == NULL) {
             return NULL;
         }
@@ -2593,7 +2593,7 @@ task_step_impl(TaskObj *task, PyObject *exc)
 
         if (!exc) {
             /* exc was not a CancelledError */
-            exc = _PyObject_CallNoArg(asyncio_CancelledError);
+            exc = PyObject_CallNoArgs(asyncio_CancelledError);
             if (!exc) {
                 goto fail;
             }
@@ -3308,7 +3308,7 @@ module_init(void)
     PyObject *weak_set;
     WITH_MOD("weakref")
     GET_MOD_ATTR(weak_set, "WeakSet");
-    all_tasks = _PyObject_CallNoArg(weak_set);
+    all_tasks = PyObject_CallNoArgs(weak_set);
     Py_CLEAR(weak_set);
     if (all_tasks == NULL) {
         goto fail;

Modules/_datetimemodule.c

@@ -3607,7 +3607,7 @@ tzinfo_reduce(PyObject *self, PyObject *Py_UNUSED(ignored))
 
     getinitargs = _PyObject_GetAttrId(self, &PyId___getinitargs__);
     if (getinitargs != NULL) {
-        args = _PyObject_CallNoArg(getinitargs);
+        args = PyObject_CallNoArgs(getinitargs);
         Py_DECREF(getinitargs);
         if (args == NULL) {
             return NULL;
@@ -3624,7 +3624,7 @@ tzinfo_reduce(PyObject *self, PyObject *Py_UNUSED(ignored))
 
     getstate = _PyObject_GetAttrId(self, &PyId___getstate__);
     if (getstate != NULL) {
-        state = _PyObject_CallNoArg(getstate);
+        state = PyObject_CallNoArgs(getstate);
         Py_DECREF(getstate);
         if (state == NULL) {
             Py_DECREF(args);

Modules/_elementtree.c

@@ -3892,7 +3892,7 @@ _elementtree_XMLParser_close_impl(XMLParserObject *self)
     }
     else if (self->handle_close) {
         Py_DECREF(res);
-        return _PyObject_CallNoArg(self->handle_close);
+        return PyObject_CallNoArgs(self->handle_close);
     }
     else {
         return res;

Modules/_pickle.c

@@ -1274,7 +1274,7 @@ _Unpickler_ReadFromFile(UnpicklerObject *self, Py_ssize_t n)
         return -1;
 
     if (n == READ_WHOLE_LINE) {
-        data = _PyObject_CallNoArg(self->readline);
+        data = PyObject_CallNoArgs(self->readline);
     }
     else {
         PyObject *len;
@@ -4411,7 +4411,7 @@ save(PicklerObject *self, PyObject *obj, int pers_save)
             /* Check for a __reduce__ method. */
             reduce_func = _PyObject_GetAttrId(obj, &PyId___reduce__);
             if (reduce_func != NULL) {
-                reduce_value = _PyObject_CallNoArg(reduce_func);
+                reduce_value = PyObject_CallNoArgs(reduce_func);
             }
             else {
                 PyErr_Format(st->PicklingError,

Modules/_tkinter.c

@@ -2768,7 +2768,7 @@ TimerHandler(ClientData clientData)
 
     ENTER_PYTHON
 
-    res = _PyObject_CallNoArg(func);
+    res = PyObject_CallNoArgs(func);
     Py_DECREF(func);
     Py_DECREF(v); /* See Tktt_New() */
 

Modules/readline.c

@@ -867,7 +867,7 @@ on_hook(PyObject *func)
     int result = 0;
     if (func != NULL) {
         PyObject *r;
-        r = _PyObject_CallNoArg(func);
+        r = PyObject_CallNoArgs(func);
         if (r == NULL)
             goto error;
         if (r == Py_None)

Objects/call.c

@@ -70,6 +70,14 @@ _Py_CheckFunctionResult(PyObject *callable, PyObject *result, const char *where)
 
 /* --- Core PyObject call functions ------------------------------- */
 
+/* Call a callable Python object without any arguments */
+PyObject *
+PyObject_CallNoArgs(PyObject *func)
+{
+    return _PyObject_CallNoArg(func);
+}
+
+
 PyObject *
 _PyObject_FastCallDict(PyObject *callable, PyObject *const *args,
                        size_t nargsf, PyObject *kwargs)