Commit

Author: StanFromIreland

Lib/test/test_traceback.py

@@ -5049,6 +5049,16 @@ def test_no_site_package_flavour(self):
                 b"add the site-packages directory to sys.path?"), stderr
         )
 
+    def test_missing_stdlib_package(self):
+        code = """
+            import sys
+            sys.stdlib_module_names |= {'spam'}
+            import spam
+        """
+        _, _, stderr = assert_python_failure('-S', '-c', code)
+
+        self.assertIn(b"Standard library module 'spam' was not found", stderr)
+
 
 class TestColorizedTraceback(unittest.TestCase):
     maxDiff = None

Lib/traceback.py

@@ -1109,11 +1109,11 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
                 self._str += f". Did you mean: '{suggestion}'?"
         elif exc_type and issubclass(exc_type, ModuleNotFoundError):
             module_name = getattr(exc_value, "name", None)
-            if sys.flags.no_site and module_name not in sys.stdlib_module_names:
+            if module_name in sys.stdlib_module_names:
+                self._str = f"Standard library module '{module_name}' was not found"
+            elif sys.flags.no_site:
                 self._str += (". Site initialization is disabled, did you forget to "
                     + "add the site-packages directory to sys.path?")
-            elif module_name in sys.stdlib_module_names:
-                self._str = f"Standard library module '{module_name}' was not found"
         elif exc_type and issubclass(exc_type, (NameError, AttributeError)) and \
                 getattr(exc_value, "name", None) is not None:
             wrong_name = getattr(exc_value, "name", None)