dynamodb2 - updated error message when UpdateItem with ConditionalExpression fails (#4375)

moto/dynamodb2/responses.py

@@ -855,9 +855,7 @@ class DynamoHandler(BaseResponse):
             return self.error(er, mve.exception_msg)
         except ValueError:
             er = "com.amazonaws.dynamodb.v20111205#ConditionalCheckFailedException"
-            return self.error(
+            return self.error(er, "The conditional request failed")
-                er, "A condition specified in the operation could not be evaluated."
-            )
         except TypeError:
             er = "com.amazonaws.dynamodb.v20111205#ValidationException"
             return self.error(er, "Validation Exception")

tests/test_dynamodb2/test_dynamodb.py

@@ -2877,7 +2877,7 @@ def test_condition_expressions():
         ExpressionAttributeValues={":match": {"S": "match"}},
     )
 
-    with pytest.raises(client.exceptions.ConditionalCheckFailedException):
+    with pytest.raises(client.exceptions.ConditionalCheckFailedException) as exc:
         client.update_item(
             TableName="test1",
             Key={"client": {"S": "client1"}, "app": {"S": "app1"}},
@@ -2886,6 +2886,18 @@ def test_condition_expressions():
             ExpressionAttributeValues={":match": {"S": "match"}},
             ExpressionAttributeNames={"#existing": "existing", "#match": "match"},
         )
+    _assert_conditional_check_failed_exception(exc)
+
+    with pytest.raises(client.exceptions.ConditionalCheckFailedException) as exc:
+        client.update_item(
+            TableName="test1",
+            Key={"client": {"S": "client2"}, "app": {"S": "app1"}},
+            UpdateExpression="set #match=:match",
+            ConditionExpression="attribute_exists(#existing)",
+            ExpressionAttributeValues={":match": {"S": "match"}},
+            ExpressionAttributeNames={"#existing": "existing", "#match": "match"},
+        )
+    _assert_conditional_check_failed_exception(exc)
 
     with pytest.raises(client.exceptions.ConditionalCheckFailedException):
         client.delete_item(
@@ -2897,6 +2909,12 @@ def test_condition_expressions():
         )
 
 
+def _assert_conditional_check_failed_exception(exc):
+    err = exc.value.response["Error"]
+    err["Code"].should.equal("ConditionalCheckFailedException")
+    err["Message"].should.equal("The conditional request failed")
+
+
 @mock_dynamodb2
 def test_condition_expression_numerical_attribute():
     dynamodb = boto3.resource("dynamodb", region_name="us-east-1")
@@ -2972,8 +2990,9 @@ def test_condition_expression__attr_doesnt_exist():
     update_if_attr_doesnt_exist()
 
     # Second time should fail
-    with pytest.raises(client.exceptions.ConditionalCheckFailedException):
+    with pytest.raises(client.exceptions.ConditionalCheckFailedException) as exc:
         update_if_attr_doesnt_exist()
+    _assert_conditional_check_failed_exception(exc)
 
 
 @mock_dynamodb2
@@ -3012,7 +3031,7 @@ def test_condition_expression__and_order():
 
     # ensure that the RHS of the AND expression is not evaluated if the LHS
     # returns true (as it would result an error)
-    with pytest.raises(client.exceptions.ConditionalCheckFailedException):
+    with pytest.raises(client.exceptions.ConditionalCheckFailedException) as exc:
         client.update_item(
             TableName="test",
             Key={"forum_name": {"S": "the-key"}},
@@ -3021,6 +3040,7 @@ def test_condition_expression__and_order():
             ExpressionAttributeNames={"#ttl": "ttl"},
             ExpressionAttributeValues={":ttl": {"N": "6"}, ":old_ttl": {"N": "5"}},
         )
+    _assert_conditional_check_failed_exception(exc)
 
 
 @mock_dynamodb2