Begins a transaction.

        This method is called automatically when entering a with
        statement, however it can be called explicitly if you don't want
        to use a context manager.

        :raises: :class:`~exceptions.ValueError` if the transaction has
                 already begun.

def begin(self):
        """Begins a transaction.

        This method is called automatically when entering a with
        statement, however it can be called explicitly if you don't want
        to use a context manager.

        :raises: :class:`~exceptions.ValueError` if the transaction has
                 already begun.
        """
        super(Transaction, self).begin()
        try:
            response_pb = self._client._datastore_api.begin_transaction(self.project)
            self._id = response_pb.transaction
        except:  # noqa: E722 do not use bare except, specify exception instead
            self._status = self._ABORTED
            raise