return Item in cancellation_reasons when transaction fails and return…

…_values=ALL_OLD (#1226)

docs/transaction.rst

@@ -94,7 +94,8 @@ Now, say you make another attempt to debit one of the accounts when they don't h
                 condition=(
                     (BankStatement.account_balance >= transfer_amount) &
                     (BankStatement.is_active == True)
-                )
+                ),
+                return_values=ALL_OLD
             )
             transaction.update(
                 BankStatement(user_id='user2'),
@@ -107,6 +108,8 @@ Now, say you make another attempt to debit one of the accounts when they don't h
         assert e.cause_response_code == 'TransactionCanceledException'
         # the first 'update' was a reason for the cancellation
         assert e.cancellation_reasons[0].code == 'ConditionalCheckFailed'
+        # when return_values=ALL_OLD, the old values can be accessed from the raw_item property
+        assert BankStatement.from_dynamodb_dict(e.cancellation_reasons[0].raw_item) == user1_statement
         # the second 'update' wasn't a reason, but was cancelled too
         assert e.cancellation_reasons[1] is None
 

pynamodb/connection/base.py

@@ -357,6 +357,7 @@ def _make_api_call(self, operation_name: str, operation_kwargs: Dict) -> Dict:
                         CancellationReason(
                             code=d['Code'],
                             message=d.get('Message'),
+                            raw_item=cast(Optional[Dict[str, Dict[str, Any]]], d.get('Item')),
                         ) if d['Code'] != 'None' else None
                     )
                     for d in cancellation_reasons

pynamodb/exceptions.py

@@ -134,6 +134,7 @@ class CancellationReason:
     """
     code: str
     message: Optional[str] = None
+    raw_item: Optional[Dict[str, Dict[str, Any]]] = None
 
 
 class TransactWriteError(PynamoDBException):

tests/integration/test_transaction_integration.py

@@ -5,6 +5,7 @@
 import pytest
 
 from pynamodb.connection import Connection
+from pynamodb.constants import ALL_OLD
 from pynamodb.exceptions import CancellationReason
 from pynamodb.exceptions import DoesNotExist, TransactWriteError, InvalidStateError
 
@@ -168,6 +169,22 @@ def test_transact_write__error__transaction_cancelled__condition_check_failure(c
     assert BankStatement.Meta.table_name in exc_info.value.cause.MSG_TEMPLATE
 
 
+@pytest.mark.ddblocal
+def test_transact_write__error__transaction_cancelled__condition_check_failure__return_all_old(connection):
+    # create a users and a bank statements for them
+    User(1).save()
+
+    # attempt to do this as a transaction with the condition that they don't already exist
+    with pytest.raises(TransactWriteError) as exc_info:
+        with TransactWrite(connection=connection) as transaction:
+            transaction.save(User(1), condition=(User.user_id.does_not_exist()), return_values=ALL_OLD)
+    assert exc_info.value.cause_response_code == TRANSACTION_CANCELLED
+    assert 'ConditionalCheckFailed' in exc_info.value.cause_response_message
+    assert exc_info.value.cancellation_reasons == [
+        CancellationReason(code='ConditionalCheckFailed', message='The conditional request failed', raw_item=User(1).to_dynamodb_dict()),
+    ]
+
+
 @pytest.mark.ddblocal
 def test_transact_write__error__transaction_cancelled__partial_failure(connection):
     User(2).delete()