gh-151631: Fix traceback when parent package is blocked in sys.modules

_find_incompatible_extension_module() must not import a parent package
blocked via sys.modules[parent] = None when formatting ModuleNotFoundError.

Author: bhuvi27

Lib/test/test_traceback.py

@@ -5375,6 +5375,30 @@ def test_incompatible_extension_modules_hint(self):
         hint = f'Although a module with this name was found for a different Python version ({incompatible_module}).'
         self.assertIn(hint, stderr.decode())
 
+    def test_module_not_found_with_blocked_parent(self):
+        # gh-151631: formatting a ModuleNotFoundError for a submodule must not
+        # raise when the parent package is blocked via sys.modules[parent]=None.
+        sys.modules['pkg'] = None
+        self.addCleanup(sys.modules.pop, 'pkg', None)
+        with self.assertRaises(ModuleNotFoundError) as cm:
+            from pkg.mod import name  # noqa: F401
+        exc = cm.exception
+
+        self.assertIsNone(
+            traceback._find_incompatible_extension_module('pkg.mod'))
+        te = traceback.TracebackException(
+            type(exc), exc, exc.__traceback__)
+        formatted = ''.join(te.format())
+        self.assertIn("No module named 'pkg.mod'", formatted)
+
+        formatted2 = ''.join(traceback.format_exception(exc))
+        self.assertIn("No module named 'pkg.mod'", formatted2)
+
+        import logging
+        with self.assertLogs('test_traceback', level='ERROR') as cm_logs:
+            logging.getLogger('test_traceback').exception('fail', exc_info=exc)
+        self.assertIn("No module named 'pkg.mod'", cm_logs.output[0])
+
 
 class TestColorizedTraceback(unittest.TestCase):
     maxDiff = None

Lib/traceback.py

@@ -2104,7 +2104,12 @@ def _find_incompatible_extension_module(module_name):
 
     parent, _, child = module_name.rpartition('.')
     if parent:
-        traversable = importlib.resources.files(parent)
+        try:
+            traversable = importlib.resources.files(parent)
+        except ImportError:
+            # gh-151631: importlib.resources.files() imports the parent
+            # package, which fails when sys.modules[parent] is None.
+            return
     else:
         traversable = importlib.resources.readers.MultiplexedPath(
             *map(pathlib.Path, filter(os.path.isdir, sys.path))

Misc/NEWS.d/next/Library/2026-06-21-10-30-00.gh-issue-151631.xK9mPq.rst

@@ -0,0 +1,3 @@
+Fix :func:`traceback.format_exception` raising :exc:`ModuleNotFoundError` when
+formatting an existing :exc:`ModuleNotFoundError` for a submodule whose parent
+package is blocked via ``sys.modules[parent] = None``.