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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def begin(self):
 """Begin a transaction on the database. :rtype: bytes :returns: the ID for the newly-begun transaction. :raises ValueError: if the transaction is already begun, committed, or rolled back. """

        if self._transaction_id is not None:
            raise ValueError("Transaction already begun")

        if self.committed is not None:
            raise ValueError("Transaction already committed")

        if self._rolled_back:
            raise ValueError("Transaction is already rolled back")

        database = self._session._database
        api = database.spanner_api
        metadata = _metadata_with_prefix(database.name)
        txn_options = TransactionOptions(read_write=TransactionOptions.ReadWrite())
        response = api.begin_transaction(
            self._session.name, txn_options, metadata=metadata
        )
        self._transaction_id = response.id
        return self._transaction_id