Returns the TransactionReceipt

        Args:
            txn_id (str): the id of the transaction for which the receipt
                should be retrieved.

        Returns:
            TransactionReceipt: The receipt for the given transaction id.

        Raises:
            KeyError: if the transaction id is unknown.

def get(self, txn_id):
        """Returns the TransactionReceipt

        Args:
            txn_id (str): the id of the transaction for which the receipt
                should be retrieved.

        Returns:
            TransactionReceipt: The receipt for the given transaction id.

        Raises:
            KeyError: if the transaction id is unknown.
        """
        if txn_id not in self._receipt_db:
            raise KeyError('Unknown transaction id {}'.format(txn_id))

        txn_receipt_bytes = self._receipt_db[txn_id]
        txn_receipt = TransactionReceipt()
        txn_receipt.ParseFromString(txn_receipt_bytes)
        return txn_receipt