Powertools for AWS Lambda (Python)を使うときに気をつけたいDynamoDB JSONのデシリアライズ

"event": {
                "type": "<class 'aws_lambda_powertools.utilities.data_classes.dynamo_db_stream_event.DynamoDBStreamEvent'>",
                "value": {
                    "Records": [
                        {
                            "eventID": "",
                            "eventName": "INSERT",
                            "eventVersion": "1.1",
                            "eventSource": "aws:dynamodb",
                            "awsRegion": "ap-northeast-1",
                            "dynamodb": {
                                "ApproximateCreationDateTime": 1722483841,
                                "Keys": {
                                    "pk": {
                                        "S": "pk_value"
                                    }
                                },
                                "SequenceNumber": "",
                                "SizeBytes": 30,
                                "StreamViewType": "KEYS_ONLY"
                            },
                            "eventSourceARN": 
                        }
                    ]
                }
            }

{"pk": {"S": "pk_value"}}

@event_source(data_class=DynamoDBStreamEvent)
def handler(
    event: DynamoDBStreamEvent,
    context,
):
    for record in event.records:
        keys = record.dynamodb.keys

        pk = keys["pk"].["S"]   # これだとエラー
        pk = keys["pk"]         # なぜかこれで取得できる

@property
def keys(self) -> Optional[Dict[str, Any]]:  # type: ignore[override]
    """The primary key attribute(s) for the DynamoDB item that was modified."""
    return self._deserialize_dynamodb_dict("Keys")

    def _deserialize_dynamodb_dict(self, key: str) -> Optional[Dict[str, Any]]:
        """Deserialize DynamoDB records available in `Keys`, `NewImage`, and `OldImage`

        Parameters
        ----------
        key : str
            DynamoDB key (e.g., Keys, NewImage, or OldImage)

        Returns
        -------
        Optional[Dict[str, Any]]
            Deserialized records in Python native types
        """
        dynamodb_dict = self._data.get(key)
        if dynamodb_dict is None:
            return None

        return {k: self._deserializer.deserialize(v) for k, v in dynamodb_dict.items()}

{"pk": {"S": "pk_value"}}  変換前
{"pk": "pk_value"}         変換後 属性を指定しなくて良くなった👍