Repository: getmoto/moto (python)
Issue: DynamoDb transact write items errors for Update expression multiple set action
When using moto mock dynamodb, transact_write_items operation errors even when there is only a single SET action in an update action to dynamo. Error as below

_**botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the TransactWriteItems operation: Invalid UpdateExpression: The "SET" section can only be used once in an update expression;**_

Issue was found in moto 5.0.10.
Reverting back to 5.0.9 works as expected.
Pull request description: DynamoDB: transact_write_items() should validate set clauses, not actions
Fixes #7817 

`SET action1, action2` is allowed. `SET action1 SET action2` is not allowed.

So when validating this, we need to count the number of clauses (SET statements)  - not the number of actions ([action1, action2]).



Patch:
diff --git a/moto/dynamodb/parsing/ast_nodes.py b/moto/dynamodb/parsing/ast_nodes.py
index 81afbfae5..95875d90d 100644
--- a/moto/dynamodb/parsing/ast_nodes.py
+++ b/moto/dynamodb/parsing/ast_nodes.py
@@ -40,7 +40,8 @@ class Node(metaclass=abc.ABCMeta):
             if len(set_attributes) != len(set(set_attributes)):
                 raise DuplicateUpdateExpression(set_attributes)
 
-            if limit_set_actions and len(set_actions) > 1:
+            set_clauses = self.find_clauses([UpdateExpressionSetClause])
+            if limit_set_actions and len(set_clauses) > 1:
                 raise MockValidationException(
                     'Invalid UpdateExpression: The "SET" section can only be used once in an update expression;'
                 )


Test patch:
diff --git a/tests/test_dynamodb/exceptions/test_dynamodb_transactions.py b/tests/test_dynamodb/exceptions/test_dynamodb_transactions.py
index 90877bd28..722f10565 100644
--- a/tests/test_dynamodb/exceptions/test_dynamodb_transactions.py
+++ b/tests/test_dynamodb/exceptions/test_dynamodb_transactions.py
@@ -85,7 +85,7 @@ def test_transact_write_items__put_and_delete_on_same_item(table_name=None):
 
 @pytest.mark.aws_verified
 @dynamodb_aws_verified(add_range=True)
-def test_transact_write_items__update_with_multiple_sets(table_name=None):
+def test_transact_write_items__update_with_multiple_set_clauses(table_name=None):
     dynamodb = boto3.client("dynamodb", region_name="us-east-1")
 
     with pytest.raises(ClientError) as exc:
@@ -112,6 +112,28 @@ def test_transact_write_items__update_with_multiple_sets(table_name=None):
         == 'Invalid UpdateExpression: The "SET" section can only be used once in an update expression;'
     )
 
+    # Note that we can do this by simply separating the statements with a comma
+    dynamodb.transact_write_items(
+        TransactItems=[
+            {
+                "Update": {
+                    "TableName": table_name,
+                    "Key": {"pk": {"S": "s"}, "sk": {"S": "t"}},
+                    "UpdateExpression": "SET a1 = :v1, a2 = :v2",
+                    "ExpressionAttributeValues": {
+                        ":v1": {"S": "v1"},
+                        ":v2": {"S": "v1"},
+                    },
+                }
+            }
+        ]
+    )
+
+    inventory = dynamodb.scan(TableName=table_name)["Items"]
+    assert inventory == [
+        {"a1": {"S": "v1"}, "sk": {"S": "t"}, "a2": {"S": "v1"}, "pk": {"S": "s"}}
+    ]
+
 
 @mock_aws
 def test_transact_write_items__too_many_transactions():