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).

def current(self):
        """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).
        """
        top = super(Transaction, self).current()
        if isinstance(top, Transaction):
            return top