bpo-45614: Fix traceback display for exceptions with invalid module name (GH-29726) (GH-29827)

(cherry picked from commit 4dfae6f38e)

Lib/test/test_traceback.py

@@ -740,6 +740,17 @@ def test_message_none(self):
         err = self.get_report(Exception(''))
         self.assertIn('Exception\n', err)
 
+    def test_exception_modulename_not_unicode(self):
+        class X(Exception):
+            def __str__(self):
+                return "I am X"
+
+        X.__module__ = 42
+
+        err = self.get_report(X())
+        exp = f'<unknown>.{X.__qualname__}: I am X\n'
+        self.assertEqual(exp, err)
+
     def test_syntax_error_various_offsets(self):
         for offset in range(-5, 10):
             for add in [0, 2]:

Lib/traceback.py

@@ -574,6 +574,8 @@ def format_exception_only(self):
         stype = self.exc_type.__qualname__
         smod = self.exc_type.__module__
         if smod not in ("__main__", "builtins"):
+            if not isinstance(smod, str):
+                smod = "<unknown>"
             stype = smod + '.' + stype
 
         if not issubclass(self.exc_type, SyntaxError):

Misc/NEWS.d/next/Core and Builtins/2021-11-23-12-06-41.bpo-45614.fIekgI.rst

@@ -0,0 +1 @@
+Fix :mod:`traceback` display for exceptions with invalid module name.

Python/pythonrun.c

@@ -903,7 +903,7 @@ print_exception(PyObject *f, PyObject *value)
         {
             Py_XDECREF(modulename);
             PyErr_Clear();
-            err = PyFile_WriteString("<unknown>", f);
+            err = PyFile_WriteString("<unknown>.", f);
         }
         else {
             if (!_PyUnicode_EqualToASCIIId(modulename, &PyId_builtins))