gh-127598: Improve ModuleNotFoundError when -S is passed

Lib/test/test_traceback.py

@@ -4748,7 +4748,39 @@ class MyList(list):
         with self.assertRaises(TypeError):
             _suggestions._generate_suggestions(MyList(), "")
 
+    @support.requires_subprocess()
+    def test_no_site_package_flavour(self):
+        import subprocess
+
+        cmd = [sys.executable, '-S', '-c', 'import boo']
+        result = subprocess.run(
+            cmd,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            text=True
+        )
+
+        self.assertNotEqual(result.returncode, 0)
+        self.assertTrue(
+            ("Site initialization is disabled, did you forget to "
+                + "add the site-packages directory to sys.path?") in result.stderr
+        )
+
+        cmd = [sys.executable, '-S', '-c',
+            'import sys; sys.builtin_module_names = sys.builtin_module_names + ("boo",); import boo']
 
+        result = subprocess.run(
+            cmd,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            text=True
+        )
+
+        self.assertNotEqual(result.returncode, 0)
+        self.assertTrue(
+            ("Site initialization is disabled, did you forget to "
+                + "add the site-packages directory to sys.path?") not in result.stderr
+        )
 
 
 class TestColorizedTraceback(unittest.TestCase):

Lib/traceback.py

@@ -1106,6 +1106,11 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
             suggestion = _compute_suggestion_error(exc_value, exc_traceback, wrong_name)
             if suggestion:
                 self._str += f". Did you mean: '{suggestion}'?"
+        elif exc_type and issubclass(exc_type, ModuleNotFoundError) and \
+                sys.flags.no_site and \
+                getattr(exc_value, "name", None) not in sys.builtin_module_names:
+            self._str += (". Site initialization is disabled, did you forget to "
+                + "add the site-packages directory to sys.path?")
         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)

Misc/NEWS.d/next/Core_and_Builtins/2025-07-19-17-08-09.gh-issue-127598.Mx8S-y.rst

@@ -0,0 +1,2 @@
+Improve :exc:`ModuleNotFoundError` by adding flavour text to exception when the
+:option:`-S` option is passed. Patch by Andrea Mattei.