It seems we could have connection=None as an optional / implicit argument in the Transaction methods add_auto_id_entity(), begin(), commit() and rollback().

On Connection, the current transaction is used to alter the behavior of the methods, e.g. from commit()

        if self.transaction():
            request.mode = datastore_pb.CommitRequest.TRANSACTIONAL
            request.transaction = self.transaction().id()
        else:
            request.mode = datastore_pb.CommitRequest.NON_TRANSACTIONAL

It seems instead this separation would benefit by having Transaction subclass Connection and adding the "extra" transaction parts when needed.