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

Lib/importlib/_bootstrap_external.py

@@ -1105,7 +1105,15 @@ 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 as e:
+            raise ModuleNotFoundError(
+                f"{parent_module_name!r} must be imported before finding {self._name!r}.",
+                name=parent_module_name,
+            ) from e
+        else:
+            return getattr(module, path_attr_name)
 
     def _recalculate(self):
         # If the parent's path has changed, recalculate _path

Lib/test/test_importlib/namespace_pkgs/foo/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/test_namespace_pkgs.py

@@ -318,6 +318,17 @@ def test_module_before_namespace_package(self):
         self.assertEqual(a_test.attr, 'in module')
 
 
+class NamespaceSubpackageSameName(NamespacePackageTest):
+    paths = ['']
+
+    def test_namespace_subpackage_shares_name_with_directory(self):
+        submodule_path = 'project4.foo'
+        with self.assertRaises(ModuleNotFoundError) as cm:
+            importlib.machinery.PathFinder.find_spec(submodule_path)
+
+        self.assertEqual(cm.exception.name, 'project4')
+
+
 class ReloadTests(NamespacePackageTest):
     paths = ['portion1']
 

Makefile.pre.in

@@ -2470,6 +2470,7 @@ TESTSUBDIRS=	idlelib/idle_test \
 		test/test_importlib/namespace_pkgs \
 		test/test_importlib/namespace_pkgs/both_portions \
 		test/test_importlib/namespace_pkgs/both_portions/foo \
+		test/test_importlib/namespace_pkgs/foo \
 		test/test_importlib/namespace_pkgs/module_and_namespace_package \
 		test/test_importlib/namespace_pkgs/module_and_namespace_package/a_test \
 		test/test_importlib/namespace_pkgs/not_a_namespace_pkg \

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

@@ -0,0 +1,3 @@
+Reraise :exc:`KeyError` as :exc:`ModuleNotFoundError` when
+:meth:`importlib.machinery.PathFinder.find_spec` is called on a submodule
+without importing the parent (and without a ``path`` argument).