Index: Lib/inspect.py
===================================================================
--- Lib/inspect.py	(Revision 88454)
+++ Lib/inspect.py	(Arbeitskopie)
@@ -1065,19 +1065,22 @@
 
 def _check_instance(obj, attr):
     instance_dict = {}
-    try:
-        instance_dict = object.__getattribute__(obj, "__dict__")
-    except AttributeError:
-        pass
-    return instance_dict.get(attr, _sentinel)
+    if not _dict_shadowed(type(obj)):
+        try:
+            instance_dict = object.__getattribute__(obj, "__dict__")
+        except AttributeError:
+            pass
+        return dict.get(instance_dict, attr, _sentinel)
+    else:
+        return _sentinel
 
-
 def _check_class(klass, attr):
     for entry in _static_getmro(klass):
-        try:
-            return entry.__dict__[attr]
-        except KeyError:
-            pass
+        if not _dict_shadowed(type(klass)):
+            try:
+                return entry.__dict__[attr]
+            except KeyError:
+                pass
     return _sentinel
 
 def _is_type(obj):
@@ -1087,6 +1090,20 @@
         return False
     return True
 
+def _dict_shadowed(klass):
+    dict_attr = type.__dict__["__dict__"]
+    for entry in _static_getmro(klass):
+        try:
+            class_dict = dict_attr.__get__(entry)["__dict__"]
+        except KeyError:
+            pass
+        else:
+            if not (type(class_dict) is types.GetSetDescriptorType and
+                    class_dict.__name__ == "__dict__" and
+                    class_dict.__objclass__ is entry):
+                return True
+    # No __dict__ attribute found at all, means it can't be shadowed
+    return False
 
 def getattr_static(obj, attr, default=_sentinel):
     """Retrieve attributes without triggering dynamic lookup via the
@@ -1121,10 +1138,11 @@
     if obj is klass:
         # for types we check the metaclass too
         for entry in _static_getmro(type(klass)):
-            try:
-                return entry.__dict__[attr]
-            except KeyError:
-                pass
+            if not _dict_shadowed(type(entry)):
+                try:
+                    return entry.__dict__[attr]
+                except KeyError:
+                    pass
     if default is not _sentinel:
         return default
     raise AttributeError(attr)
Index: Lib/test/test_inspect.py
===================================================================
--- Lib/test/test_inspect.py	(Revision 88454)
+++ Lib/test/test_inspect.py	(Arbeitskopie)
@@ -906,7 +906,58 @@
         self.assertEqual(inspect.getattr_static(Something(), 'foo'), 3)
         self.assertEqual(inspect.getattr_static(Something, 'foo'), 3)
 
+    def test_custom_instance_dict(self):
+        sentinel = object()
 
+        class MyDict(dict):
+            def get(self, key, default=sentinel):
+                self["executed"] = True
+                if default is sentinel:
+                    return super().get(key)
+                else:
+                    return super().get(key, default)
+
+        class Thing(object):
+            def __init__(self):
+                self.__dict__ = MyDict()
+                self.__dict__["executed"] = False
+                self.__dict__["key"] = 42
+
+        instance = Thing()
+        self.assertEqual(inspect.getattr_static(instance, "key"), 42)
+        self.assertFalse(instance.executed)
+
+    def test_dict_as_property(self):
+        class Thing(object):
+            spam = 42
+            executed = False
+
+            @property
+            def __dict__(self):
+                self.executed = True
+                return dict(spam=23)
+
+        instance = Thing()
+        self.assertEquals(inspect.getattr_static(instance, "spam"), 42)
+        self.assertFalse(instance.executed)
+
+    def test_metaclass_dict_as_property(self):
+        class Meta(type):
+            @property
+            def __dict__(self):
+                self.executed = True
+
+        class Thing(metaclass=Meta):
+            executed = False
+
+            def __init__(self):
+                self.spam = 42
+
+        instance = Thing()
+        self.assertEquals(inspect.getattr_static(instance, "spam"), 42)
+        self.assertFalse(Thing.executed)
+
+
 class TestGetGeneratorState(unittest.TestCase):
 
     def setUp(self):