gh-149313: Preserve exception notes/cause in dict/set unhashable type…

Lib/test/test_dict.py

@@ -1630,6 +1630,26 @@ def __hash__(self):
         with self.assertRaises(KeyError):
             d.get(key2)
 
+    def test_unhashable_key_preserves_exception_info(self):
+        """gh-149313: exception notes and cause must survive the improved message."""
+        class Key:
+            def __hash__(self):
+                try:
+                    hash([])
+                except TypeError as e:
+                    e.add_note("custom note")
+                    raise
+
+        with self.assertRaises(TypeError) as cm:
+            {Key(): 1}
+
+        exc = cm.exception
+        self.assertIn("cannot use", str(exc))
+        # The original exception is chained as __cause__
+        self.assertIsNotNone(exc.__cause__)
+        self.assertIn("unhashable type: 'list'", str(exc.__cause__))
+        self.assertIn("custom note", exc.__cause__.__notes__[0])
+
     def test_clear_at_lookup(self):
         # gh-140551 dict crash if clear is called at lookup stage
         class X:

Lib/test/test_set.py

@@ -686,6 +686,25 @@ def __hash__(self):
         with self.assertRaises(KeyError):
             myset.discard(elem2)
 
+    def test_unhashable_element_preserves_exception_info(self):
+        """gh-149313: exception notes and cause must survive the improved message."""
+        class Elem:
+            def __hash__(self):
+                try:
+                    hash([])
+                except TypeError as e:
+                    e.add_note("custom note")
+                    raise
+
+        with self.assertRaises(TypeError) as cm:
+            {Elem()}
+
+        exc = cm.exception
+        self.assertIn("cannot use", str(exc))
+        self.assertIsNotNone(exc.__cause__)
+        self.assertIn("unhashable type: 'list'", str(exc.__cause__))
+        self.assertIn("custom note", exc.__cause__.__notes__[0])
+
     def test_hash_collision_remove_add(self):
         self.maxDiff = None
         # There should be enough space, so all elements with unique hash

Misc/NEWS.d/next/Core_and_Builtins/2026-05-07-23-09-44.gh-issue-149313.MnAD9T.rst

@@ -0,0 +1,3 @@
+Preserve original exception's ``__notes__``, ``__traceback__``, and
+``__cause__`` when rewriting :exc:`TypeError` for unhashable dict keys and
+set elements.

Objects/dictobject.c

@@ -2416,7 +2416,11 @@ dict_unhashable_type(PyObject *op, PyObject *key)
         errmsg = "cannot use '%T' as a dict key (%S)";
     }
     PyErr_Format(PyExc_TypeError, errmsg, key, exc);
+    PyObject *exc2 = PyErr_GetRaisedException();
+    PyException_SetCause(exc2, Py_NewRef(exc));
+    PyException_SetContext(exc2, Py_NewRef(exc));
     Py_DECREF(exc);
+    PyErr_SetRaisedException(exc2);
 }
 
 Py_ssize_t

Objects/setobject.c

@@ -355,7 +355,11 @@ set_unhashable_type(PyObject *key)
     PyErr_Format(PyExc_TypeError,
                  "cannot use '%T' as a set element (%S)",
                  key, exc);
+    PyObject *exc2 = PyErr_GetRaisedException();
+    PyException_SetCause(exc2, Py_NewRef(exc));
+    PyException_SetContext(exc2, Py_NewRef(exc));
     Py_DECREF(exc);
+    PyErr_SetRaisedException(exc2);
 }
 
 int