How to use the placebo.serializer.Format.ALLOWED function in placebo

next_file = None
        serializer_format = None
        index = self._index.setdefault(base_name, 1)

        while not next_file:
            file_name = os.path.join(
                self._data_path, base_name + '_{0}'.format(index))
            next_file, serializer_format = self.find_file_format(file_name)
            if next_file:
                self._index[base_name] += 1
            elif index != 1:
                index = 1
                self._index[base_name] = 1
            else:
                raise IOError('response file ({0}.[{1}]) not found'.format(
                    file_name, "|".join(Format.ALLOWED)))

        return next_file, serializer_format

def __init__(self, prefix=None, debug=False, record_format=Format.JSON):
        if debug:
            self._set_logger(__name__, logging.DEBUG)

        record_format = record_format.lower()
        if record_format not in Format.ALLOWED:
            LOG.warning("Record format not allowed. Falling back to default.")
            record_format = Format.DEFAULT

        self._serializer = get_serializer(record_format)
        self._filename_re = re.compile(r'.*\..*_(?P\d+).{0}'.format(
            record_format))
        self._record_format = record_format
        self.prefix = prefix
        self._uuid = str(uuid.uuid4())
        self._data_path = None
        self._mode = None
        self._session = None
        self._index = {}
        self.events = []
        self.clients = []

def find_file_format(file_name):
        """
        Returns a tuple with the file path and format found, or (None, None)
        """
        for file_format in Format.ALLOWED:
            file_path = '.'.join((file_name, file_format))
            if os.path.exists(file_path):
                return file_path, file_format
        return None, None