From: MonadChains <monadchains@gmail.com>
Date: Sun, 20 Jul 2025 13:33:58 +0000 (+0100)
Subject: gh-127598: Improve ModuleNotFoundError when -S is passed (GH-136821)
X-Git-Tag: v3.15.0a1~919
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=18a7f5dad832488e8965fe29159a62abc18368e2;p=thirdparty%2FPython%2Fcpython.git

gh-127598: Improve ModuleNotFoundError when -S is passed (GH-136821)
---

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 74b979d00966..11b7f419bddb 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -4748,7 +4748,26 @@ class MiscTest(unittest.TestCase):
         with self.assertRaises(TypeError):
             _suggestions._generate_suggestions(MyList(), "")
 
+    def test_no_site_package_flavour(self):
+        code = """import boo"""
+        _, _, stderr = assert_python_failure('-S', '-c', code)
 
+        self.assertIn(
+            (b"Site initialization is disabled, did you forget to "
+                b"add the site-packages directory to sys.path?"), stderr
+        )
+
+        code = """
+            import sys
+            sys.stdlib_module_names = sys.stdlib_module_names + ("boo",)
+            import boo
+        """
+        _, _, stderr = assert_python_failure('-S', '-c', code)
+
+        self.assertNotIn(
+            (b"Site initialization is disabled, did you forget to "
+                b"add the site-packages directory to sys.path?"), stderr
+        )
 
 
 class TestColorizedTraceback(unittest.TestCase):
diff --git a/Lib/traceback.py b/Lib/traceback.py
index 31aa8695735f..f0dbb6352f77 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -1106,6 +1106,11 @@ class TracebackException:
             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.stdlib_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)
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-07-19-17-08-09.gh-issue-127598.Mx8S-y.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-07-19-17-08-09.gh-issue-127598.Mx8S-y.rst
new file mode 100644
index 000000000000..aff047bbef0f
--- /dev/null
+++ b/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 the exception when the
+:option:`-S` option is passed. Patch by Andrea Mattei.