:param data_key: The name of the dict key in the external representation, i.e.
        the input of `load` and the output of `dump`.
        If `None`, the key will match the name of the field.
    :param attribute: The name of the key/attribute in the internal representation, i.e.
        the output of `load` and the input of `dump`.
        If `None`, the key/attribute will match the name of the field.
        Note: This should only be used for very specific use cases such as
        outputting multiple fields for a single attribute, or using keys/attributes
        that are invalid variable names, unsuitable for field names. In most cases,
        you should use ``data_key`` instead.