Exposing control over return_values in Model.update()

pynamodb/models.py

@@ -46,7 +46,7 @@
     ATTR_DEFINITIONS, ATTR_NAME, ATTR_TYPE, KEY_SCHEMA,
     KEY_TYPE, ITEM, READ_CAPACITY_UNITS, WRITE_CAPACITY_UNITS,
     RANGE_KEY, ATTRIBUTES, PUT, DELETE, RESPONSES,
-    INDEX_NAME, PROVISIONED_THROUGHPUT, PROJECTION, ALL_NEW,
+    INDEX_NAME, PROVISIONED_THROUGHPUT, PROJECTION, ALL_NEW, NONE,
     GLOBAL_SECONDARY_INDEXES, LOCAL_SECONDARY_INDEXES, KEYS,
     PROJECTION_TYPE, NON_KEY_ATTRIBUTES,
     TABLE_STATUS, ACTIVE, RETURN_VALUES, BATCH_GET_PAGE_LIMIT,
@@ -417,31 +417,43 @@ def delete(self, condition: Optional[Condition] = None, settings: OperationSetti
 
         return self._get_connection().delete_item(hk_value, range_key=rk_value, condition=condition, settings=settings)
 
-    def update(self, actions: List[Action], condition: Optional[Condition] = None, settings: OperationSettings = OperationSettings.default) -> Any:
+    def update(
+        self,
+        actions: List[Action],
+        condition: Optional[Condition] = None,
+        settings: OperationSettings = OperationSettings.default,
+        read_back: str = ALL_NEW,
+    ) -> Any:
         """
         Updates an item using the UpdateItem operation.
 
         :param actions: a list of Action updates to apply
         :param condition: an optional Condition on which to update
         :param settings: per-operation settings
+        :param read_back: what to read back from the DB after the update
+          ALL_NEW: read back entire object after the update (default)
+          NONE: read back nothing, local version is not updated
         :raises ModelInstance.DoesNotExist: if the object to be updated does not exist
         :raises pynamodb.exceptions.UpdateError: if the `condition` is not met
         """
         if not isinstance(actions, list) or len(actions) == 0:
             raise TypeError("the value of `actions` is expected to be a non-empty list")
+        if read_back not in (ALL_NEW, NONE):
+            raise ValueError("expected `read_back` to be `ALL_NEW` or `NONE`, but was: {}".format(read_back))
 
         hk_value, rk_value = self._get_hash_range_key_serialized_values()
         version_condition = self._handle_version_attribute(actions=actions)
         if version_condition is not None:
             condition &= version_condition
 
-        data = self._get_connection().update_item(hk_value, range_key=rk_value, return_values=ALL_NEW, condition=condition, actions=actions, settings=settings)
-        item_data = data[ATTRIBUTES]
-        stored_cls = self._get_discriminator_class(item_data)
-        if stored_cls and stored_cls != type(self):
-            raise ValueError("Cannot update this item from the returned class: {}".format(stored_cls.__name__))
-        self.deserialize(item_data)
-        return data
+        data = self._get_connection().update_item(hk_value, range_key=rk_value, return_values=read_back, condition=condition, actions=actions, settings=settings)
+        if read_back == ALL_NEW:
+            item_data = data[ATTRIBUTES]
+            stored_cls = self._get_discriminator_class(item_data)
+            if stored_cls and stored_cls != type(self):
+                raise ValueError("Cannot update this item from the returned class: {}".format(stored_cls.__name__))
+            self.deserialize(item_data)
+            return data
 
     def save(self, condition: Optional[Condition] = None, settings: OperationSettings = OperationSettings.default) -> Dict[str, Any]:
         """

tests/test_model.py

@@ -17,7 +17,7 @@
 from pynamodb.constants import (
     ITEM, STRING, ALL, KEYS_ONLY, INCLUDE, REQUEST_ITEMS, UNPROCESSED_KEYS, CAMEL_COUNT,
     RESPONSES, KEYS, ITEMS, LAST_EVALUATED_KEY, EXCLUSIVE_START_KEY, ATTRIBUTES, BINARY,
-    UNPROCESSED_ITEMS, DEFAULT_ENCODING, MAP, LIST, NUMBER, SCANNED_COUNT,
+    UNPROCESSED_ITEMS, DEFAULT_ENCODING, MAP, LIST, NUMBER, SCANNED_COUNT, ALL_NEW, NONE
 )
 from pynamodb.models import Model
 from pynamodb.indexes import (
@@ -921,6 +921,28 @@ def test_update(self, mock_time):
             assert item.views is None
             self.assertEqual({'bob'}, item.custom_aliases)
 
+    def test_update_readback(self):
+        self.init_table_meta(SimpleUserModel, SIMPLE_MODEL_TABLE_DATA)
+        item = SimpleUserModel(user_name='foo', is_active=True, email='[email protected]', signature='foo', views=100)
+
+        with patch(PATCH_METHOD) as req:
+            req.return_value = {}
+            item.update(
+                actions=[SimpleUserModel.email.set('[email protected]')],
+                read_back=NONE)
+            params = {
+                'TableName': 'SimpleModel',
+                'Key': {'user_name': {'S': 'foo'}},
+                'ReturnValues': 'NONE',
+                'UpdateExpression': 'SET #0 = :0',
+                'ExpressionAttributeNames': {'#0': 'email'},
+                'ExpressionAttributeValues': {':0': {'S': '[email protected]'}},
+                'ReturnConsumedCapacity': 'TOTAL'
+            }
+            args = req.call_args[0][1]
+            deep_eq(args, params, _assert=True)
+            assert item.email == '[email protected]'
+
     def test_update_doesnt_do_validation_on_null_attributes(self):
         self.init_table_meta(CarModel, CAR_MODEL_TABLE_DATA)
         item = CarModel(12345)