Implement Exactly-Once Transaction Log in Python

from dataclasses import dataclass
from typing import Dict, Optional


@dataclass
class TxnRecord:
    txn_id: str
    status: str


class ExactlyOnceTxnLog:
    def __init__(self) -> None:
        self._log: Dict[str, TxnRecord] = {}
        self._processed_ids: set = set()

    def record(self, txn_id: str, status: str) -> bool:
        """Record a transaction exactly once. Returns True if newly recorded."""
        if txn_id in self._processed_ids:
            return False
        self._processed_ids.add(txn_id)
        self._log[txn_id] = TxnRecord(txn_id=txn_id, status=status)
        return True

    def get(self, txn_id: str) -> Optional[TxnRecord]:
        return self._log.get(txn_id)


if __name__ == "__main__":
    log = ExactlyOnceTxnLog()
    print(log.record("txn-001", "committed"))
    print(log.record("txn-001", "duplicate-attempt"))
    print(log.record("txn-002", "failed"))
    print(log.get("txn-001"))