PyErr_SetObject adds note to exception raised on normalization error

Include/cpython/pyerrors.h

@@ -112,6 +112,10 @@ PyAPI_FUNC(PyObject *) _PyErr_FormatFromCause(
 
 /* In exceptions.c */
 
+PyAPI_FUNC(PyObject*) _PyException_AddNote(
+     PyBaseExceptionObject *exc,
+     PyObject *note);
+
 /* Helper that attempts to replace the current exception with one of the
  * same type but with a prefix added to the exception text. The resulting
  * exception description looks like:

Lib/test/test_capi/test_exceptions.py

@@ -169,5 +169,25 @@ class Broken(Exception, metaclass=Meta):
         with self.assertRaises(ZeroDivisionError) as e:
             _testcapi.exc_set_object(Broken, Broken())
 
+    def test_set_object_and_fetch(self):
+        class Broken(Exception):
+            def __init__(self, *arg):
+                 raise ValueError("Broken __init__")
+
+        exc = _testcapi.exc_set_object_fetch(Broken, ('abcd'))
+        self.assertIsInstance(exc, ValueError)
+        self.assertEqual(exc.__notes__[0],
+                         "Normalization failed: type=Broken args='abcd'")
+
+        class BadArg:
+            def __repr__(self):
+                raise TypeError('Broken arg type')
+
+        exc = _testcapi.exc_set_object_fetch(Broken, (BadArg()))
+        self.assertIsInstance(exc, ValueError)
+        self.assertEqual(exc.__notes__[0],
+                         'Normalization failed: type=Broken args=<unknown>')
+
+
 if __name__ == "__main__":
     unittest.main()

Modules/_testcapi/exceptions.c

@@ -92,6 +92,26 @@ exc_set_object(PyObject *self, PyObject *args)
     return NULL;
 }
 
+static PyObject *
+exc_set_object_fetch(PyObject *self, PyObject *args)
+{
+    PyObject *exc;
+    PyObject *obj;
+    PyObject *type;
+    PyObject *value;
+    PyObject *tb;
+
+    if (!PyArg_ParseTuple(args, "OO:exc_set_object", &exc, &obj)) {
+        return NULL;
+    }
+
+    PyErr_SetObject(exc, obj);
+    PyErr_Fetch(&type, &value, &tb);
+    Py_XDECREF(type);
+    Py_XDECREF(tb);
+    return value;
+}
+
 static PyObject *
 raise_exception(PyObject *self, PyObject *args)
 {
@@ -262,6 +282,7 @@ static PyMethodDef test_methods[] = {
     {"make_exception_with_doc", _PyCFunction_CAST(make_exception_with_doc),
      METH_VARARGS | METH_KEYWORDS},
     {"exc_set_object",          exc_set_object,                  METH_VARARGS},
+    {"exc_set_object_fetch",    exc_set_object_fetch,            METH_VARARGS},
     {"raise_exception",         raise_exception,                 METH_VARARGS},
     {"raise_memoryerror",       raise_memoryerror,               METH_NOARGS},
     {"set_exc_info",            test_set_exc_info,               METH_VARARGS},

Objects/exceptions.c

@@ -3749,6 +3749,12 @@ _PyExc_Fini(PyInterpreterState *interp)
     _PyExc_FiniTypes(interp);
 }
 
+PyObject *
+_PyException_AddNote(PyBaseExceptionObject *exc, PyObject *note)
+{
+    return BaseException_add_note((PyObject *)exc, note);
+}
+
 /* Helper to do the equivalent of "raise X from Y" in C, but always using
  * the current exception rather than passing one in.
  *

Python/errors.c

@@ -135,6 +135,28 @@ _PyErr_GetTopmostException(PyThreadState *tstate)
     return exc_info;
 }
 
+PyObject *
+get_normalization_failure_note(PyThreadState *tstate, PyObject *exception, PyObject *value)
+{
+    PyObject *args = PyObject_Repr(value);
+    if (args == NULL) {
+        _PyErr_Clear(tstate);
+        args = PyUnicode_FromFormat("<unknown>");
+    }
+    PyObject *note;
+    const char *tpname = ((PyTypeObject*)exception)->tp_name;
+    if (args == NULL) {
+        _PyErr_Clear(tstate);
+        note = PyUnicode_FromFormat("Normalization failed: type=%s", tpname);
+    }
+    else {
+        note = PyUnicode_FromFormat("Normalization failed: type=%s args=%S",
+                                    tpname, args);
+        Py_DECREF(args);
+    }
+    return note;
+}
+
 void
 _PyErr_SetObject(PyThreadState *tstate, PyObject *exception, PyObject *value)
 {
@@ -169,6 +191,16 @@ _PyErr_SetObject(PyThreadState *tstate, PyObject *exception, PyObject *value)
         fixed_value = _PyErr_CreateException(exception, value);
         Py_XDECREF(value);
         if (fixed_value == NULL) {
+            PyObject *exc = _PyErr_GetRaisedException(tstate);
+            assert(PyExceptionInstance_Check(exc));
+
+            PyObject *note = get_normalization_failure_note(tstate, exception, value);
+            if (note != NULL) {
+                PyObject *res = _PyException_AddNote((PyBaseExceptionObject*)exc, note);
+                Py_DECREF(note);
+                Py_XDECREF(res);
+            }
+            _PyErr_SetRaisedException(tstate, exc);
             return;
         }