Module aws_lambda_powertools.shared.dynamodb_deserializer

Classes

class TypeDeserializer

Deserializes DynamoDB types to Python types.

It's based on boto3's DynamoDB TypeDeserializer.

The only notable difference is that for Binary (B, BS) values we return Python Bytes directly, since we don't support Python 2.

Expand source code

class TypeDeserializer:
    """
    Deserializes DynamoDB types to Python types.

    It's based on boto3's [DynamoDB TypeDeserializer](https://boto3.amazonaws.com/v1/documentation/api/latest/_modules/boto3/dynamodb/types.html).

    The only notable difference is that for Binary (`B`, `BS`) values we return Python Bytes directly,
    since we don't support Python 2.
    """

    def deserialize(self, value: dict) -> Any:
        """Deserialize DynamoDB data types into Python types.

        Parameters
        ----------
        value: Any
            DynamoDB value to be deserialized to a python type


            Here are the various conversions:

            DynamoDB                                Python
            --------                                ------
            {'NULL': True}                          None
            {'BOOL': True/False}                    True/False
            {'N': Decimal(value)}                   Decimal(value)
            {'S': string}                           string
            {'B': bytes}                            bytes
            {'NS': [str(value)]}                    set([str(value)])
            {'SS': [string]}                        set([string])
            {'BS': [bytes]}                         set([bytes])
            {'L': list}                             list
            {'M': dict}                             dict

        Parameters
        ----------
        value: Any
            DynamoDB value to be deserialized to a python type

        Returns
        --------
        any
            Python native type converted from DynamoDB type
        """

        dynamodb_type = list(value.keys())[0]
        deserializer: Callable | None = getattr(self, f"_deserialize_{dynamodb_type}".lower(), None)
        if deserializer is None:
            raise TypeError(f"Dynamodb type {dynamodb_type} is not supported")

        return deserializer(value[dynamodb_type])

    def _deserialize_null(self, value: bool) -> None:
        return None

    def _deserialize_bool(self, value: bool) -> bool:
        return value

    def _deserialize_n(self, value: str) -> Decimal:
        # value is None or "."? It's zero
        # then return early
        value = value.lstrip("0")
        if not value or value == ".":
            return DYNAMODB_CONTEXT.create_decimal(0)

        if len(value) > 38:
            # See: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/HowItWorks.NamingRulesDataTypes.html#HowItWorks.DataTypes.Number
            # Calculate the number of trailing zeros after the 38th character
            tail = len(value[38:]) - len(value[38:].rstrip("0"))
            # Trim the value: remove trailing zeros if any, or just take the first 38 characters
            value = value[:-tail] if tail > 0 else value[:38]

        return DYNAMODB_CONTEXT.create_decimal(value)

    def _deserialize_s(self, value: str) -> str:
        return value

    def _deserialize_b(self, value: bytes) -> bytes:
        return value

    def _deserialize_ns(self, value: Sequence[str]) -> set[Decimal]:
        return set(map(self._deserialize_n, value))

    def _deserialize_ss(self, value: Sequence[str]) -> set[str]:
        return set(map(self._deserialize_s, value))

    def _deserialize_bs(self, value: Sequence[bytes]) -> set[bytes]:
        return set(map(self._deserialize_b, value))

    def _deserialize_l(self, value: Sequence[dict]) -> Sequence[Any]:
        return [self.deserialize(v) for v in value]

    def _deserialize_m(self, value: dict) -> dict:
        return {k: self.deserialize(v) for k, v in value.items()}

Methods

def deserialize(self, value: dict) ‑> Any

Deserialize DynamoDB data types into Python types.

Parameters

value : Any

DynamoDB value to be deserialized to a python type

Here are the various conversions:

DynamoDB Python -------- ------ {'NULL': True} None {'BOOL': True/False} True/False {'N': Decimal(value)} Decimal(value) {'S': string} string {'B': bytes} bytes {'NS': [str(value)]} set([str(value)]) {'SS': [string]} set([string]) {'BS': [bytes]} set([bytes]) {'L': list} list {'M': dict} dict

Parameters

value : Any

DynamoDB value to be deserialized to a python type

Returns

any

Python native type converted from DynamoDB type