More conservative interpretation of PathFinder.find_spec() failures

astroid/interpreter/_import/util.py

@@ -34,8 +34,17 @@ def is_namespace(modname: str) -> bool:
                 working_modname, path=last_submodule_search_locations
             )
         except ValueError:
-            # Assume it's a .pth file, unless it's __main__
-            return modname != "__main__"
+            if modname == "__main__":
+                return False
+            try:
+                # .pth files will be on sys.modules
+                return sys.modules[modname].__spec__ is None
+            except KeyError:
+                return False
+            except AttributeError:
+                # Workaround for "py" module
+                # https://github.com/pytest-dev/apipkg/issues/13
+                return False
         except KeyError:
             # Intermediate steps might raise KeyErrors
             # https://github.com/python/cpython/issues/93334

tests/unittest_manager.py

@@ -129,6 +129,15 @@ def test_module_is_not_namespace(self) -> None:
         self.assertFalse(util.is_namespace("tests.testdata.python3.data.all"))
         self.assertFalse(util.is_namespace("__main__"))
 
+    def test_module_unexpectedly_missing_spec(self) -> None:
+        astroid_module = sys.modules["astroid"]
+        original_spec = astroid_module.__spec__
+        del astroid_module.__spec__
+        try:
+            self.assertFalse(util.is_namespace("astroid"))
+        finally:
+            astroid_module.__spec__ = original_spec
+
     def test_implicit_namespace_package(self) -> None:
         data_dir = os.path.dirname(resources.find("data/namespace_pep_420"))
         contribute = os.path.join(data_dir, "contribute_to_namespace")