gh-159693: Format import errors with no usable search paths

Lib/test/test_traceback.py

@@ -5325,6 +5325,20 @@ def test_windows_only_module_error(self):
         else:
             self.fail("ModuleNotFoundError was not raised")
 
+    def test_module_not_found_without_search_paths(self):
+        importlib.import_module('importlib.resources.readers')
+        error = ModuleNotFoundError("No module named 'missing_module'",
+                                    name='missing_module')
+        with os_helper.temp_dir() as tmp:
+            for paths in ([], [os.path.join(tmp, 'missing')]):
+                with self.subTest(paths=paths):
+                    with support.swap_attr(sys, 'path', paths):
+                        self.assertEqual(
+                            traceback.format_exception_only(error),
+                            ["ModuleNotFoundError: No module named "
+                             "'missing_module'\n"],
+                        )
+
     @unittest.skipIf(not importlib.machinery.EXTENSION_SUFFIXES, 'Platform does not support extension modules')
     def test_find_incompatible_extension_modules(self):
         """_find_incompatible_extension_modules assumes the last extension in

Lib/traceback.py

@@ -2106,9 +2106,10 @@ def _find_incompatible_extension_module(module_name):
     if parent:
         traversable = importlib.resources.files(parent)
     else:
-        traversable = importlib.resources.readers.MultiplexedPath(
-            *map(pathlib.Path, filter(os.path.isdir, sys.path))
-        )
+        paths = list(map(pathlib.Path, filter(os.path.isdir, sys.path)))
+        if not paths:
+            return
+        traversable = importlib.resources.readers.MultiplexedPath(*paths)
 
     for entry in traversable.iterdir():
         if entry.name.startswith(child + '.') and entry.name.endswith(untagged_suffix):

Misc/NEWS.d/next/Library/2026-10-20-02-01-00.gh-issue-159693.6Rky2a.rst

@@ -0,0 +1,2 @@
+Fix formatting of :exc:`ModuleNotFoundError` when :data:`sys.path` contains
+no existing directories, including when displaying errors in the REPL.