Special case __main__ in is_namespace() (#1579)

Author: jacobtylerwalls

astroid/interpreter/_import/util.py

@@ -34,8 +34,8 @@ def is_namespace(modname: str) -> bool:
                 working_modname, path=last_submodule_search_locations
             )
         except ValueError:
-            # executed .pth files may not have __spec__
-            return True
+            # Assume it's a .pth file, unless it's __main__
+            return modname != "__main__"
         except KeyError:
             # Intermediate steps might raise KeyErrors
             # https://github.com/python/cpython/issues/93334

tests/unittest_manager.py

@@ -127,6 +127,7 @@ def test_submodule_homonym_with_non_module(self) -> None:
 
     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_implicit_namespace_package(self) -> None:
         data_dir = os.path.dirname(resources.find("data/namespace_pep_420"))