From 0093a7992fe6d833055b4b63a5ac6652aa245604 Mon Sep 17 00:00:00 2001
From: Garrett Heel <garrettheel@users.noreply.github.com>
Date: Thu, 11 Jul 2019 08:27:05 -0400
Subject: [PATCH] dynamodb2: Defer evaluation of the OR RHS in condition expr

---
 moto/dynamodb2/comparisons.py         |  3 +--
 tests/test_dynamodb2/test_dynamodb.py | 30 +++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/moto/dynamodb2/comparisons.py b/moto/dynamodb2/comparisons.py
index 1a4633e64..151a314f1 100644
--- a/moto/dynamodb2/comparisons.py
+++ b/moto/dynamodb2/comparisons.py
@@ -1004,8 +1004,7 @@ class OpOr(Op):
 
     def expr(self, item):
         lhs = self.lhs.expr(item)
-        rhs = self.rhs.expr(item)
-        return lhs or rhs
+        return lhs or self.rhs.expr(item)
 
 
 class Func(object):
diff --git a/tests/test_dynamodb2/test_dynamodb.py b/tests/test_dynamodb2/test_dynamodb.py
index f5afc1e7e..7746cf66b 100644
--- a/tests/test_dynamodb2/test_dynamodb.py
+++ b/tests/test_dynamodb2/test_dynamodb.py
@@ -1964,6 +1964,36 @@ def test_condition_expression__attr_doesnt_exist():
         update_if_attr_doesnt_exist()
 
 
+@mock_dynamodb2
+def test_condition_expression__or_order():
+    client = boto3.client('dynamodb', region_name='us-east-1')
+
+    client.create_table(
+        TableName='test',
+        KeySchema=[{'AttributeName': 'forum_name', 'KeyType': 'HASH'}],
+        AttributeDefinitions=[
+            {'AttributeName': 'forum_name', 'AttributeType': 'S'},
+        ],
+        ProvisionedThroughput={'ReadCapacityUnits': 1, 'WriteCapacityUnits': 1},
+    )
+
+    # ensure that the RHS of the OR expression is not evaluated if the LHS
+    # returns true (as it would result an error)
+    client.update_item(
+        TableName='test',
+        Key={
+            'forum_name': {'S': 'the-key'},
+        },
+        UpdateExpression='set #ttl=:ttl',
+        ConditionExpression='attribute_not_exists(#ttl) OR #ttl <= :old_ttl',
+        ExpressionAttributeNames={'#ttl': 'ttl'},
+        ExpressionAttributeValues={
+            ':ttl': {'N': '6'},
+            ':old_ttl': {'N': '5'},
+        }
+    )
+
+
 @mock_dynamodb2
 def test_query_gsi_with_range_key():
     dynamodb = boto3.client('dynamodb', region_name='us-east-1')