bpo-38530: Make sure that failing to generate suggestions on failure will not propagate exceptions (GH-25408)

Lib/test/test_exceptions.py

@@ -1695,6 +1695,20 @@ class AttributeErrorTests(unittest.TestCase):
 
             self.assertIn("blech", err.getvalue())
 
+    def test_attribute_error_with_failing_dict(self):
+        class T:
+            bluch = 1
+            def __dir__(self):
+                raise AttributeError("oh no!")
+
+        try:
+            T().blich
+        except AttributeError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertNotIn("blech", err.getvalue())
+        self.assertNotIn("oh no!", err.getvalue())
 
 class ImportErrorTests(unittest.TestCase):
 

Python/pythonrun.c

@@ -962,6 +962,8 @@ print_exception(PyObject *f, PyObject *value)
             err += PyFile_WriteString("?", f);
         }
         Py_DECREF(suggestions);
+    } else if (PyErr_Occurred()) {
+        PyErr_Clear();
     }
     err += PyFile_WriteString("\n", f);
     Py_XDECREF(tb);

Python/suggestions.c

@@ -89,14 +89,12 @@ calculate_suggestions(PyObject *dir,
     PyObject *suggestion = NULL;
     const char *name_str = PyUnicode_AsUTF8(name);
     if (name_str == NULL) {
-        PyErr_Clear();
         return NULL;
     }
     for (int i = 0; i < dir_size; ++i) {
         PyObject *item = PyList_GET_ITEM(dir, i);
         const char *item_str = PyUnicode_AsUTF8(item);
         if (item_str == NULL) {
-            PyErr_Clear();
             return NULL;
         }
         Py_ssize_t current_distance = levenshtein_distance(name_str, item_str);
@@ -156,7 +154,6 @@ offer_suggestions_for_name_error(PyNameErrorObject *exc) {
     assert(code != NULL && code->co_varnames != NULL);
     PyObject *dir = PySequence_List(code->co_varnames);
     if (dir == NULL) {
-        PyErr_Clear();
         return NULL;
     }
 
@@ -168,7 +165,6 @@ offer_suggestions_for_name_error(PyNameErrorObject *exc) {
 
     dir = PySequence_List(frame->f_globals);
     if (dir == NULL) {
-        PyErr_Clear();
         return NULL;
     }
     suggestions = calculate_suggestions(dir, name);
@@ -178,16 +174,16 @@ offer_suggestions_for_name_error(PyNameErrorObject *exc) {
 }
 
 // Offer suggestions for a given exception. Returns a python string object containing the
-// suggestions. This function does not raise exceptions and returns NULL if no suggestion was found.
+// suggestions. This function returns NULL if no suggestion was found or if an exception happened,
+// users must call PyErr_Occurred() to disambiguate.
 PyObject *_Py_Offer_Suggestions(PyObject *exception) {
     PyObject *result = NULL;
-    assert(!PyErr_Occurred()); // Check that we are not going to clean any existing exception
+    assert(!PyErr_Occurred());
     if (PyErr_GivenExceptionMatches(exception, PyExc_AttributeError)) {
         result = offer_suggestions_for_attribute_error((PyAttributeErrorObject *) exception);
     } else if (PyErr_GivenExceptionMatches(exception, PyExc_NameError)) {
         result = offer_suggestions_for_name_error((PyNameErrorObject *) exception);
     }
-    assert(!PyErr_Occurred());
     return result;
 }