gh-93334: Fix homonym edge case in PathFinder.find_spec()

Lib/importlib/_bootstrap_external.py

@@ -1367,7 +1367,11 @@ def _find_parent_path_names(self):
 
     def _get_parent_path(self):
         parent_module_name, path_attr_name = self._find_parent_path_names()
-        return getattr(sys.modules[parent_module_name], path_attr_name)
+        try:
+            module = sys.modules[parent_module_name]
+        except KeyError:
+            return sys.path
+        return getattr(module, path_attr_name)
 
     def _recalculate(self):
         # If the parent's path has changed, recalculate _path

Lib/test/test_importlib/namespace_pkgs/homonym/README.md

@@ -0,0 +1,2 @@
+This directory should not be a package, but should share a name with an
+unrelated subpackage.

Lib/test/test_importlib/namespace_pkgs/project4/homonym.py

@@ -0,0 +1 @@
+attr = 'homonym'

Lib/test/test_importlib/test_namespace_pkgs.py

@@ -319,6 +319,17 @@ def test_module_before_namespace_package(self):
         self.assertEqual(a_test.attr, 'in module')
 
 
+class DirectoryHomonym(NamespacePackageTest):
+    paths = ['']
+
+    def test_namespace_package_submodule_homonym(self):
+        submodule_path = 'project4.homonym'
+        got = importlib.machinery.PathFinder.find_spec(submodule_path)
+        self.assertEqual(got.name, submodule_path)
+        self.assertIsNone(got.loader)
+        self.assertNotIn('project4', got.submodule_search_locations[0])
+
+
 class ReloadTests(NamespacePackageTest):
     paths = ['portion1']
 

Misc/NEWS.d/next/Library/2022-10-08-14-56-07.gh-issue-93334.0KUm8d.rst

@@ -0,0 +1,3 @@
+Prevent :exc:`KeyError` thrown by :meth:`importlib.machinery.PathFinder.find_spec`
+when a directory that is not a module shares a name with a namespace package
+submodule, and where a ``path`` argument is not provided.