2 files changed, 24 insertions, 9 deletions
diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
index 8949365..6b2221d 100644
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -1724,10 +1724,13 @@ def _setup(sys_module, _imp_module):
         BYTECODE_SUFFIXES = DEBUG_BYTECODE_SUFFIXES
 
     module_type = type(sys)
-    for module in sys.modules.values():
+    for name, module in sys.modules.items():
         if isinstance(module, module_type):
             if not hasattr(module, '__loader__'):
-                module.__loader__ = BuiltinImporter
+                if name in sys.builtin_module_names:
+                    module.__loader__ = BuiltinImporter
+                elif _imp.is_frozen(name):
+                    module.__loader__ = FrozenImporter
 
     self_module = sys.modules[__name__]
     for builtin_name in ('_io', '_warnings', 'builtins', 'marshal'):
diff --git a/Lib/test/test_importlib/test_api.py b/Lib/test/test_importlib/test_api.py
index ef6629a..b39ab09 100644
--- a/Lib/test/test_importlib/test_api.py
+++ b/Lib/test/test_importlib/test_api.py
@@ -1,9 +1,11 @@
 from . import util
 import imp
 import importlib
+from importlib import _bootstrap
 from importlib import machinery
 import sys
 from test import support
+import types
 import unittest
 
 
@@ -175,13 +177,23 @@ class FrozenImportlibTests(unittest.TestCase):
                                     machinery.FrozenImporter))
 
 
-def test_main():
-    from test.support import run_unittest
-    run_unittest(ImportModuleTests,
-                 FindLoaderTests,
-                 InvalidateCacheTests,
-                 FrozenImportlibTests)
+class StartupTests(unittest.TestCase):
+
+    def test_everyone_has___loader__(self):
+        # Issue #17098: all modules should have __loader__ defined.
+        for name, module in sys.modules.items():
+            if isinstance(module, types.ModuleType):
+                self.assertTrue(hasattr(module, '__loader__'),
+                        '{!r} lacks a __loader__ attribute'.format(name))
+                if name in sys.builtin_module_names:
+                    self.assertIn(module.__loader__,
+                            (importlib.machinery.BuiltinImporter,
+                             importlib._bootstrap.BuiltinImporter))
+                elif imp.is_frozen(name):
+                    self.assertIn(module.__loader__,
+                            (importlib.machinery.FrozenImporter,
+                             importlib._bootstrap.FrozenImporter))
 
 
 if __name__ == '__main__':
-    test_main()
+    unittest.main()