gh-125916: Adapt functools.reduce() to Argument Clinic

Lib/test/test_inspect/test_inspect.py

@@ -5708,8 +5708,8 @@ def test_faulthandler_module_has_signatures(self):
         self._test_module_has_signatures(faulthandler, unsupported_signature=unsupported_signature)
 
     def test_functools_module_has_signatures(self):
-        no_signature = {'reduce'}
-        self._test_module_has_signatures(functools, no_signature)
+        unsupported_signature = {"reduce"}
+        self._test_module_has_signatures(functools, unsupported_signature=unsupported_signature)
 
     def test_gc_module_has_signatures(self):
         import gc

Modules/_functoolsmodule.c

@@ -932,15 +932,31 @@ _functools_cmp_to_key_impl(PyObject *module, PyObject *mycmp)
 
 /* reduce (used to be a builtin) ********************************************/
 
-// Not converted to argument clinic, because of `args` in-place modification.
-// AC will affect performance.
+/*[clinic input]
+_functools.reduce
+
+    function as func: object
+    iterable as seq: object
+    initial as result: object = NULL
+    /
+
+Apply a function of two arguments cumulatively to the items of an iterable, from left to right.
+
+This effectively reduces the iterable to a single value.  If initial is present,
+it is placed before the items of the iterable in the calculation, and serves as
+a default when the iterable is empty.
+
+For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])
+calculates ((((1 + 2) + 3) + 4) + 5).
+[clinic start generated code]*/
+
 static PyObject *
-functools_reduce(PyObject *self, PyObject *args)
+_functools_reduce_impl(PyObject *module, PyObject *func, PyObject *seq,
+                       PyObject *result)
+/*[clinic end generated code: output=30d898fe1267c79d input=d233c2670cba7f66]*/
 {
-    PyObject *seq, *func, *result = NULL, *it;
+    PyObject *args, *it;
 
-    if (!PyArg_UnpackTuple(args, "reduce", 2, 3, &func, &seq, &result))
-        return NULL;
     if (result != NULL)
         Py_INCREF(result);
 
@@ -1006,18 +1022,6 @@ functools_reduce(PyObject *self, PyObject *args)
     return NULL;
 }
 
-PyDoc_STRVAR(functools_reduce_doc,
-"reduce(function, iterable[, initial], /) -> value\n\
-\n\
-Apply a function of two arguments cumulatively to the items of an iterable, from left to right.\n\
-\n\
-This effectively reduces the iterable to a single value.  If initial is present,\n\
-it is placed before the items of the iterable in the calculation, and serves as\n\
-a default when the iterable is empty.\n\
-\n\
-For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])\n\
-calculates ((((1 + 2) + 3) + 4) + 5).");
-
 /* lru_cache object **********************************************************/
 
 /* There are four principal algorithmic differences from the pure python version:
@@ -1722,7 +1726,7 @@ PyDoc_STRVAR(_functools_doc,
 "Tools that operate on functions.");
 
 static PyMethodDef _functools_methods[] = {
-    {"reduce",          functools_reduce,     METH_VARARGS, functools_reduce_doc},
+    _FUNCTOOLS_REDUCE_METHODDEF
     _FUNCTOOLS_CMP_TO_KEY_METHODDEF
     {NULL,              NULL}           /* sentinel */
 };