FIX pass the __dict__ item of a class __dict__

Author: pierreglaser

cloudpickle/cloudpickle.py

@@ -642,10 +642,23 @@ def save_dynamic_class(self, obj):
                 for k in obj.__slots__:
                     clsdict.pop(k, None)
 
-        # If type overrides __dict__ as a property, include it in the type
-        # kwargs. In Python 2, we can't set this attribute after construction.
+        # A class __dict__ is part of the class state. At unpickling time, it
+        # must be *initialized* (in an empty state)  during class creation and
+        # updated during class re-hydratation.
+        # However, a class __dict__ is read-only, and does not support direct
+        # item assignement. Instead, way to update a class __dict__  is to call
+        # setattr(k, v) on the underlying class, which has the same effect.
+        # There is one corner case: if the __dict__ class has itself a
+        # "__dict__" key (this means that the class likely overrides the
+        # __dict__ property of its instances), setattr("__dict__", v) will try
+        # to modify the read-only class __dict__ instead, and fail. As a
+        # result, if it exists, the class __dict__ must contain its __dict__
+        # item when it is initialized and fed to the class reconstructor.
         __dict__ = clsdict.pop('__dict__', None)
-        if isinstance(__dict__, property):
+        if __dict__ is not None:
+            # Native pickle memoization of dict objects prevents us from
+            # reference cycles even if __dict__ is now saved before obj is
+            # memoized.
             type_kwargs['__dict__'] = __dict__
 
         save = self.save

tests/cloudpickle_test.py

@@ -1867,6 +1867,29 @@ def __getattr__(self, name):
         with pytest.raises(pickle.PicklingError, match='recursion'):
             cloudpickle.dumps(a)
 
+    def test___dict__attribute_not_dropped_during_pickling(self):
+        # Test https://github.com/cloudpipe/cloudpickle/issues/282. cloudpickle
+        # used to drop __dict__ attributes of classes at pickling time.
+        pickle_filename = os.path.join(self.tmpdir, 'class_with___dict__.pkl')
+        _dict = {'some_attribute': 1}
+        class A:
+            __dict__ = _dict
+        a = A()
+        self.assertEqual(a.__dict__, _dict)
+
+        with open(pickle_filename, "wb") as f:
+            cloudpickle.dump(a, f, protocol=self.protocol)
+
+        # Depickle the class in a new python session to make sure the class is
+        # fully-recreated, and not looked-up in existing cloudpickle
+        # class-tracking constructs.
+        child_process_script = """
+            import pickle
+            with open("{filename}", "rb") as f:
+                depickled_a = pickle.load(f)
+            assert depickled_a.__dict__ == {_dict}, depickled_a.__dict__
+        """.format(filename=pickle_filename, _dict=_dict)
+        assert_run_python_script(textwrap.dedent(child_process_script))
 
 class Protocol2CloudPickleTest(CloudPickleTest):