don't allow LazyConfigValue on private names

avoids populating Config objects with `_repr_foo_`, etc.

IPython/config/loader.py

@@ -246,11 +246,13 b' class Config(dict):'
                 c = Config()
                 dict.__setitem__(self, key, c)
                 return c
-            else:
+            elif not key.startswith('_'):
                 # undefined, create lazy value, used for container methods
                 v = LazyConfigValue()
                 dict.__setitem__(self, key, v)
                 return v
+            else:
+                raise KeyError
 
     def __setitem__(self, key, value):
         if _is_section_key(key):

IPython/config/tests/test_loader.py

@@ -369,6 +369,14 b' class TestConfig(TestCase):'
         assert isinstance(foo, LazyConfigValue)
         self.assertIn('foo', cfg)
 
+    def test_getattr_private_missing(self):
+        cfg = Config()
+        self.assertNotIn('_repr_html_', cfg)
+        with self.assertRaises(AttributeError):
+            _ = cfg._repr_html_
+        self.assertNotIn('_repr_html_', cfg)
+        self.assertEqual(len(cfg), 0)
+
     def test_getitem_not_section(self):
         cfg = Config()
         self.assertNotIn('foo', cfg)