gh-125028: Prohibit placeholders in partial keywords

Lib/functools.py

@@ -326,6 +326,8 @@ def _partial_new(cls, func, /, *args, **keywords):
                             "or a descriptor")
     if args and args[-1] is Placeholder:
         raise TypeError("trailing Placeholders are not allowed")
+    if keywords and Placeholder in keywords.values():
+        raise TypeError("Placeholder cannot be passed as a keyword argument")
     if isinstance(func, base_cls):
         pto_phcount = func._phcount
         tot_args = func.args

Lib/test/test_functools.py

@@ -249,6 +249,11 @@ def test_placeholders_optimization(self):
         self.assertEqual(p2.args, (PH, 0))
         self.assertEqual(p2(1), ((1, 0), {}))
 
+    def test_placeholders_kw_restriction(self):
+        PH = self.module.Placeholder
+        with self.assertRaisesRegex(TypeError, "Placeholder"):
+            self.partial(capture, a=PH)
+
     def test_construct_placeholder_singleton(self):
         PH = self.module.Placeholder
         tp = type(PH)

Misc/NEWS.d/next/Library/2024-10-28-06-54-22.gh-issue-125028.GEY8Ws.rst

@@ -0,0 +1 @@
+:data:`functools.Placeholder` is restricted for :func:`functools.partial` keyword vaules.

Modules/_functoolsmodule.c

@@ -281,29 +281,31 @@ partial_new(PyTypeObject *type, PyObject *args, PyObject *kw)
     }
 
     if (pto_kw == NULL || PyDict_GET_SIZE(pto_kw) == 0) {
-        if (kw == NULL) {
-            pto->kw = PyDict_New();
-        }
-        else if (Py_REFCNT(kw) == 1) {
-            pto->kw = Py_NewRef(kw);
-        }
-        else {
-            pto->kw = PyDict_Copy(kw);
-        }
+        pto->kw = PyDict_New();
     }
     else {
         pto->kw = PyDict_Copy(pto_kw);
-        if (kw != NULL && pto->kw != NULL) {
-            if (PyDict_Merge(pto->kw, kw, 1) != 0) {
-                Py_DECREF(pto);
-                return NULL;
-            }
-        }
     }
     if (pto->kw == NULL) {
         Py_DECREF(pto);
         return NULL;
     }
+    if (kw != NULL) {
+        PyObject *key, *val;
+        Py_ssize_t pos = 0;
+        while (PyDict_Next(kw, &pos, &key, &val)) {
+            if (val == phold) {
+                Py_DECREF(pto);
+                PyErr_SetString(PyExc_TypeError,
+                                "Placeholder cannot be passed as a keyword argument");
+                return NULL;
+            }
+            if (PyDict_SetItem(pto->kw, key, val) < 0) {
+                Py_DECREF(pto);
+                return NULL;
+            }
+        }
+    }
 
     partial_setvectorcall(pto);
     return (PyObject *)pto;