Fixed issue 3448 for DynamoDB update_item (#3463)

* Fixed issue 3448 for DynamoDB update_item

* Tidied up fix for issue 3448 for DynamoDB update_item

* Reformatted fix for issue 3448 for DynamoDB update_item

* removed use of f-strings in test case as it fails in Travis CI build due to Python 2.7 support of f strings

moto/dynamodb2/responses.py

@@ -813,7 +813,6 @@ class DynamoHandler(BaseResponse):
             item_dict["Attributes"] = self._build_updated_new_attributes(
                 existing_attributes, item_dict["Attributes"]
             )
-
         return dynamo_json_dump(item_dict)
 
     def _build_updated_new_attributes(self, original, changed):
@@ -838,10 +837,8 @@ class DynamoHandler(BaseResponse):
                         )
                         for index in range(len(changed))
                     ]
-            elif changed != original:
-                return changed
             else:
-                return None
+                return changed
 
     def describe_limits(self):
         return json.dumps(

tests/test_dynamodb2/test_dynamodb.py

@@ -2437,6 +2437,44 @@ def test_update_return_attributes():
         r = update("col1", "val6", "WRONG")
 
 
+# https://github.com/spulec/moto/issues/3448
+@mock_dynamodb2
+def test_update_return_updated_new_attributes_when_same():
+    dynamo_client = boto3.resource("dynamodb", region_name="us-east-1")
+    dynamo_client.create_table(
+        TableName="moto-test",
+        KeySchema=[{"AttributeName": "HashKey1", "KeyType": "HASH"}],
+        AttributeDefinitions=[{"AttributeName": "HashKey1", "AttributeType": "S"}],
+        ProvisionedThroughput={"ReadCapacityUnits": 1, "WriteCapacityUnits": 1},
+    )
+
+    dynamodb_table = dynamo_client.Table("moto-test")
+    dynamodb_table.put_item(
+        Item={"HashKey1": "HashKeyValue1", "listValuedAttribute1": ["a", "b"]}
+    )
+
+    def update(col, to, rv):
+        return dynamodb_table.update_item(
+            TableName="moto-test",
+            Key={"HashKey1": "HashKeyValue1"},
+            UpdateExpression="SET listValuedAttribute1=:" + col,
+            ExpressionAttributeValues={":" + col: to},
+            ReturnValues=rv,
+        )
+
+    r = update("a", ["a", "c"], "UPDATED_NEW")
+    assert r["Attributes"] == {"listValuedAttribute1": ["a", "c"]}
+
+    r = update("a", {"a", "c"}, "UPDATED_NEW")
+    assert r["Attributes"] == {"listValuedAttribute1": {"a", "c"}}
+
+    r = update("a", {1, 2}, "UPDATED_NEW")
+    assert r["Attributes"] == {"listValuedAttribute1": {1, 2}}
+
+    with pytest.raises(ClientError) as ex:
+        r = update("a", ["a", "c"], "WRONG")
+
+
 @mock_dynamodb2
 def test_put_return_attributes():
     dynamodb = boto3.client("dynamodb", region_name="us-east-1")