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

Lib/test/test_traceback.py

@@ -1254,6 +1254,17 @@ class BaseExceptionReportingTests:
                 exp = "%s: %s\n" % (str_name, str_value)
                 self.assertEqual(exp, 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_exception_bad__str__(self):
         class X(Exception):
             def __str__(self):

Lib/traceback.py

@@ -808,6 +808,8 @@ class TracebackException:
         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

@@ -1022,7 +1022,7 @@ print_exception(struct exception_print_context *ctx, PyObject *value)
             {
                 Py_XDECREF(modulename);
                 PyErr_Clear();
-                err = PyFile_WriteString("<unknown>", f);
+                err = PyFile_WriteString("<unknown>.", f);
             }
             else {
                 if (!_PyUnicode_EqualToASCIIId(modulename, &PyId_builtins) &&