python · serhiy-storchaka · Mar 26, 2026
diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
@@ -311,7 +311,7 @@ PyAPI_FUNC(int) _PyEval_ExceptionGroupMatch(_PyInterpreterFrame *, PyObject* exc
 PyAPI_FUNC(void) _PyEval_FormatAwaitableError(PyThreadState *tstate, PyTypeObject *type, int oparg);
 PyAPI_FUNC(void) _PyEval_FormatExcCheckArg(PyThreadState *tstate, PyObject *exc, const char *format_str, PyObject *obj);
 PyAPI_FUNC(void) _PyEval_FormatExcUnbound(PyThreadState *tstate, PyCodeObject *co, int oparg);
-PyAPI_FUNC(void) _PyEval_FormatKwargsError(PyThreadState *tstate, PyObject *func, PyObject *kwargs);
+PyAPI_FUNC(void) _PyEval_FormatKwargsError(PyThreadState *tstate, PyObject *func, PyObject *kwargs, PyObject *dupkey);
 PyAPI_FUNC(PyObject *) _PyEval_ImportFrom(PyThreadState *, PyObject *, PyObject *);
 
 PyAPI_FUNC(PyObject *) _PyEval_LazyImportName(

diff --git a/Include/internal/pycore_dict.h b/Include/internal/pycore_dict.h
@@ -55,7 +55,7 @@ extern Py_ssize_t _PyDict_SizeOf_LockHeld(PyDictObject *);
    of a key wins, if override is 2, a KeyError with conflicting key as
    argument is raised.
 */
-PyAPI_FUNC(int) _PyDict_MergeEx(PyObject *mp, PyObject *other, int override);
+PyAPI_FUNC(int) _PyDict_MergeUniq(PyObject *mp, PyObject *other, PyObject **dupkey);
 
 extern void _PyDict_DebugMallocStats(FILE *out);
 

diff --git a/Lib/test/test_extcall.py b/Lib/test/test_extcall.py
@@ -634,11 +634,11 @@ def test_errors_in_keys():
       ...
     AttributeError: some error
 
-    >>> exc = TypeError('some error')
+    >>> exc = KeyError('some error')
     >>> f(**D())
     Traceback (most recent call last):
       ...
-    TypeError: some error
+    KeyError: 'some error'
     """
 
 def test_errors_in_keys_next():
@@ -666,11 +666,11 @@ def test_errors_in_keys_next():
       ...
     AttributeError: some error
 
-    >>> exc = TypeError('some error')
+    >>> exc = KeyError('some error')
     >>> f(**D())
     Traceback (most recent call last):
       ...
-    TypeError: some error
+    KeyError: 'some error'
     """
 
 def test_errors_in_getitem():
@@ -694,11 +694,11 @@ def test_errors_in_getitem():
       ...
     AttributeError: some error
 
-    >>> exc = TypeError('some error')
+    >>> exc = KeyError('some error')
     >>> f(**D())
     Traceback (most recent call last):
       ...
-    TypeError: some error
+    KeyError: 'some error'
     """
 
 import doctest

diff --git a/Lib/test/test_unpack_ex.py b/Lib/test/test_unpack_ex.py
@@ -681,11 +681,11 @@ def test_errors_in_keys():
       ...
     AttributeError: some error
 
-    >>> exc = TypeError('some error')
+    >>> exc = KeyError('some error')
     >>> {**D()}
     Traceback (most recent call last):
       ...
-    TypeError: some error
+    KeyError: 'some error'
     """
 
 def test_errors_in_keys_next():
@@ -712,11 +712,11 @@ def test_errors_in_keys_next():
       ...
     AttributeError: some error
 
-    >>> exc = TypeError('some error')
+    >>> exc = KeyError('some error')
     >>> {**D()}
     Traceback (most recent call last):
       ...
-    TypeError: some error
+    KeyError: 'some error'
     """
 
 def test_errors_in_getitem():
@@ -739,11 +739,11 @@ def test_errors_in_getitem():
       ...
     AttributeError: some error
 
-    >>> exc = TypeError('some error')
+    >>> exc = KeyError('some error')
     >>> {**D()}
     Traceback (most recent call last):
       ...
-    TypeError: some error
+    KeyError: 'some error'
     """
 
 __test__ = {'doctests' : doctests}

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-03-13-12-24-17.gh-issue-145876.LWFO2K.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-13-12-24-17.gh-issue-145876.LWFO2K.rst
@@ -1,3 +1,3 @@
-:exc:`AttributeError`\ s raised in :meth:`!keys` or :meth:`!__getitem__`
+:exc:`AttributeError`\ s and :exc:`KeyError`\ s raised in :meth:`!keys` or :meth:`!__getitem__`
 during dictionary unpacking (``{**mymapping}`` or ``func(**mymapping)``) are
 no longer masked by :exc:`TypeError`.
diff --git a/Modules/_testinternalcapi/test_cases.c.h b/Modules/_testinternalcapi/test_cases.c.h
@@ -139,7 +139,7 @@ As a consequence of this, split keys have a maximum size of 16.
 static PyObject* frozendict_new(PyTypeObject *type, PyObject *args,
                                 PyObject *kwds);
 static PyObject* dict_new(PyTypeObject *type, PyObject *args, PyObject *kwds);
-static int dict_merge(PyObject *a, PyObject *b, int override);
+static int dict_merge(PyObject *a, PyObject *b, int override, PyObject **dupkey);
 static int dict_contains(PyObject *op, PyObject *key);
 static int dict_merge_from_seq2(PyObject *d, PyObject *seq2, int override);
 
@@ -3391,7 +3391,7 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
             Py_DECREF(d);
             return NULL;
         }
-        if (dict_merge(copy, d, 1) < 0) {
+        if (dict_merge(copy, d, 1, NULL) < 0) {
             Py_DECREF(d);
             Py_DECREF(copy);
             return NULL;
@@ -3887,14 +3887,14 @@ static int
 dict_update_arg(PyObject *self, PyObject *arg)
 {
     if (PyAnyDict_CheckExact(arg)) {
-        return dict_merge(self, arg, 1);
+        return dict_merge(self, arg, 1, NULL);
     }
     int has_keys = PyObject_HasAttrWithError(arg, &_Py_ID(keys));
     if (has_keys < 0) {
         return -1;
     }
     if (has_keys) {
-        return dict_merge(self, arg, 1);
+        return dict_merge(self, arg, 1, NULL);
     }
     return dict_merge_from_seq2(self, arg, 1);
 }
@@ -3915,7 +3915,7 @@ dict_update_common(PyObject *self, PyObject *args, PyObject *kwds,
 
     if (result == 0 && kwds != NULL) {
         if (PyArg_ValidateKeywordArguments(kwds))
-            result = dict_merge(self, kwds, 1);
+            result = dict_merge(self, kwds, 1, NULL);
         else
             result = -1;
     }
@@ -4059,7 +4059,7 @@ PyDict_MergeFromSeq2(PyObject *d, PyObject *seq2, int override)
 }
 
 static int
-dict_dict_merge(PyDictObject *mp, PyDictObject *other, int override)
+dict_dict_merge(PyDictObject *mp, PyDictObject *other, int override, PyObject **dupkey)
 {
     assert(can_modify_dict(mp));
     ASSERT_DICT_LOCKED(other);
@@ -4068,10 +4068,10 @@ dict_dict_merge(PyDictObject *mp, PyDictObject *other, int override)
         /* a.update(a) or a.update({}); nothing to do */
         return 0;
     if (mp->ma_used == 0) {
-        /* Since the target dict is empty, PyDict_GetItem()
-            * always returns NULL.  Setting override to 1
-            * skips the unnecessary test.
-            */
+        /* Since the target dict is empty, _PyDict_Contains_KnownHash()
+         * always returns 0.  Setting override to 1
+         * skips the unnecessary test.
+         */
         override = 1;
         PyDictKeysObject *okeys = other->ma_keys;
 
@@ -4131,11 +4131,10 @@ dict_dict_merge(PyDictObject *mp, PyDictObject *other, int override)
                 err = insertdict(mp, Py_NewRef(key), hash, Py_NewRef(value));
             }
             else if (err > 0) {
-                if (override != 0) {
-                    _PyErr_SetKeyError(key);
+                if (dupkey != NULL) {
+                    *dupkey = key;
                     Py_DECREF(value);
-                    Py_DECREF(key);
-                    return -1;
+                    return -2;
                 }
                 err = 0;
             }
@@ -4155,7 +4154,7 @@ dict_dict_merge(PyDictObject *mp, PyDictObject *other, int override)
 }
 
 static int
-dict_merge(PyObject *a, PyObject *b, int override)
+dict_merge(PyObject *a, PyObject *b, int override, PyObject **dupkey)
 {
     assert(a != NULL);
     assert(b != NULL);
@@ -4167,7 +4166,7 @@ dict_merge(PyObject *a, PyObject *b, int override)
         PyDictObject *other = (PyDictObject*)b;
         int res;
         Py_BEGIN_CRITICAL_SECTION2(a, b);
-        res = dict_dict_merge((PyDictObject *)a, other, override);
+        res = dict_dict_merge((PyDictObject *)a, other, override, dupkey);
         ASSERT_CONSISTENT(a);
         Py_END_CRITICAL_SECTION2();
         return res;
@@ -4202,15 +4201,18 @@ dict_merge(PyObject *a, PyObject *b, int override)
                 status = dict_contains(a, key);
                 if (status != 0) {
                     if (status > 0) {
-                        if (override == 0) {
+                        if (dupkey == NULL) {
                             Py_DECREF(key);
                             continue;
                         }
-                        _PyErr_SetKeyError(key);
+                        *dupkey = key;
+                        res = -2;
+                    }
+                    else {
+                        Py_DECREF(key);
+                        res = -1;
                     }
-                    Py_DECREF(key);
                     Py_DECREF(iter);
-                    res = -1;
                     goto slow_exit;
                 }
             }
@@ -4246,7 +4248,7 @@ dict_merge(PyObject *a, PyObject *b, int override)
 }
 
 static int
-dict_merge_api(PyObject *a, PyObject *b, int override)
+dict_merge_api(PyObject *a, PyObject *b, int override, PyObject **dupkey)
 {
     /* We accept for the argument either a concrete dictionary object,
      * or an abstract "mapping" object.  For the former, we can do
@@ -4262,26 +4264,26 @@ dict_merge_api(PyObject *a, PyObject *b, int override)
         }
         return -1;
     }
-    return dict_merge(a, b, override);
+    return dict_merge(a, b, override, dupkey);
 }
 
 int
 PyDict_Update(PyObject *a, PyObject *b)
 {
-    return dict_merge_api(a, b, 1);
+    return dict_merge_api(a, b, 1, NULL);
 }
 
 int
 PyDict_Merge(PyObject *a, PyObject *b, int override)
 {
     /* XXX Deprecate override not in (0, 1). */
-    return dict_merge_api(a, b, override != 0);
+    return dict_merge_api(a, b, override != 0, NULL);
 }
 
 int
-_PyDict_MergeEx(PyObject *a, PyObject *b, int override)
+_PyDict_MergeUniq(PyObject *a, PyObject *b, PyObject **dupkey)
 {
-    return dict_merge_api(a, b, override);
+    return dict_merge_api(a, b, 2, dupkey);
 }
 
 /*[clinic input]
@@ -4421,7 +4423,7 @@ copy_lock_held(PyObject *o, int as_frozendict)
     }
     if (copy == NULL)
         return NULL;
-    if (dict_merge(copy, o, 1) == 0)
+    if (dict_merge(copy, o, 1, NULL) == 0)
         return copy;
     Py_DECREF(copy);
     return NULL;

@@ -2414,10 +2414,12 @@ dummy_func(
             PyObject *callable_o = PyStackRef_AsPyObjectBorrow(callable);
             PyObject *dict_o = PyStackRef_AsPyObjectBorrow(dict);
             PyObject *update_o = PyStackRef_AsPyObjectBorrow(update);
+            PyObject *dupkey = NULL;
 
-            int err = _PyDict_MergeEx(dict_o, update_o, 2);
+            int err = _PyDict_MergeUniq(dict_o, update_o, &dupkey);
             if (err < 0) {
-                _PyEval_FormatKwargsError(tstate, callable_o, update_o);
+                _PyEval_FormatKwargsError(tstate, callable_o, update_o, dupkey);
+                Py_XDECREF(dupkey);
                 ERROR_NO_POP();
             }
             u = update;

@@ -3443,9 +3443,18 @@ _Py_Check_ArgsIterable(PyThreadState *tstate, PyObject *func, PyObject *args)
 }
 
 void
-_PyEval_FormatKwargsError(PyThreadState *tstate, PyObject *func, PyObject *kwargs)
-{
-    /* _PyDict_MergeEx raises attribute
+_PyEval_FormatKwargsError(PyThreadState *tstate, PyObject *func, PyObject *kwargs, PyObject *dupkey)
+{
+    if (dupkey != NULL) {
+        PyObject *funcstr = _PyObject_FunctionStr(func);
+        _PyErr_Format(
+            tstate, PyExc_TypeError,
+            "%V got multiple values for keyword argument '%S'",
+            funcstr, "finction", dupkey);
+        Py_XDECREF(funcstr);
+        return;
+    }
+    /* _PyDict_MergeUniq raises attribute
      * error (percolated from an attempt
      * to get 'keys' attribute) instead of
      * a type error if its second argument
@@ -3465,27 +3474,6 @@ _PyEval_FormatKwargsError(PyThreadState *tstate, PyObject *func, PyObject *kwarg
             _PyErr_ChainExceptions1Tstate(tstate, exc);
         }
     }
-    else if (_PyErr_ExceptionMatches(tstate, PyExc_KeyError)) {
-        PyObject *exc = _PyErr_GetRaisedException(tstate);
-        PyObject *args = PyException_GetArgs(exc);
-        if (PyTuple_Check(args) && PyTuple_GET_SIZE(args) == 1) {
-            _PyErr_Clear(tstate);
-            PyObject *funcstr = _PyObject_FunctionStr(func);
-            if (funcstr != NULL) {
-                PyObject *key = PyTuple_GET_ITEM(args, 0);
-                _PyErr_Format(
-                    tstate, PyExc_TypeError,
-                    "%U got multiple values for keyword argument '%S'",
-                    funcstr, key);
-                Py_DECREF(funcstr);
-            }
-            Py_XDECREF(exc);
-        }
-        else {
-            _PyErr_SetRaisedException(tstate, exc);
-        }
-        Py_DECREF(args);
-    }
 }
 
 void

diff --git a/Python/executor_cases.c.h b/Python/executor_cases.c.h
diff --git a/Python/generated_cases.c.h b/Python/generated_cases.c.h