code: def current(self):
          
          top = super(Transaction, self).current()
          if isinstance(top, Transaction):
              return top
docs: Return the topmost transaction.
  
          .. note::
  
              If the topmost element on the stack is not a transaction,
              returns None.
  
          :rtype: :class:`google.cloud.datastore.transaction.Transaction` or None
          :returns: The current transaction (if any are active).