DynamoDB: empty update expression now returns a ValidationException (#…

…7989)

moto/dynamodb/responses.py

@@ -889,12 +889,22 @@ def update_item(self) -> str:
         name = self.body["TableName"]
         key = self.body["Key"]
         return_values = self.body.get("ReturnValues", "NONE")
-        update_expression = self.body.get("UpdateExpression", "").strip()
+        update_expression = self.body.get("UpdateExpression", None)
         attribute_updates = self.body.get("AttributeUpdates")
-        if update_expression and attribute_updates:
+        if update_expression is not None and attribute_updates:
             raise MockValidationException(
                 "Can not use both expression and non-expression parameters in the same request: Non-expression parameters: {AttributeUpdates} Expression parameters: {UpdateExpression}"
             )
+
+        if update_expression is not None:
+            update_expression = update_expression.strip()
+            if update_expression == "":
+                raise MockValidationException(
+                    "Invalid UpdateExpression: The expression can not be empty;"
+                )
+        else:
+            update_expression = ""
+
         return_values_on_condition_check_failure = self.body.get(
             "ReturnValuesOnConditionCheckFailure"
         )

tests/test_dynamodb/test_dynamodb.py

@@ -3864,6 +3864,31 @@ def test_error_when_providing_expression_and_nonexpression_params():
     )
 
 
+@mock_aws
+def test_error_when_providing_empty_update_expression():
+    client = boto3.client("dynamodb", "eu-central-1")
+    table_name = "testtable"
+    client.create_table(
+        TableName=table_name,
+        KeySchema=[{"AttributeName": "pkey", "KeyType": "HASH"}],
+        AttributeDefinitions=[{"AttributeName": "pkey", "AttributeType": "S"}],
+        BillingMode="PAY_PER_REQUEST",
+    )
+
+    with pytest.raises(ClientError) as ex:
+        client.update_item(
+            TableName=table_name,
+            Key={"pkey": {"S": "testrecord"}},
+            UpdateExpression="",
+            ExpressionAttributeValues={":order": {"SS": ["item"]}},
+        )
+    err = ex.value.response["Error"]
+    assert err["Code"] == "ValidationException"
+    assert (
+        err["Message"] == "Invalid UpdateExpression: The expression can not be empty;"
+    )
+
+
 @mock_aws
 def test_attribute_item_delete():
     name = "TestTable"

tests/test_dynamodb/test_dynamodb_update_expressions.py

@@ -206,3 +206,22 @@ def test_update_item_with_empty_values(table_name=None):
     err = exc.value.response["Error"]
     assert err["Code"] == "ValidationException"
     assert err["Message"] == "ExpressionAttributeValues must not be empty"
+
+
+@pytest.mark.aws_verified
+@dynamodb_aws_verified()
+def test_update_item_with_empty_expression(table_name=None):
+    dynamodb = boto3.client("dynamodb", "us-east-1")
+
+    dynamodb.put_item(TableName=table_name, Item={"pk": {"S": "foo"}})
+    with pytest.raises(ClientError) as exc:
+        dynamodb.update_item(
+            TableName=table_name,
+            Key={"pk": {"S": "foo"}},
+            UpdateExpression="",
+        )
+    err = exc.value.response["Error"]
+    assert err["Code"] == "ValidationException"
+    assert (
+        err["Message"] == "Invalid UpdateExpression: The expression can not be empty;"
+    )