def __init__(self, algorithm, length, info, backend):
        if not isinstance(backend, HMACBackend):
            raise UnsupportedAlgorithm(
                "Backend object does not implement HMACBackend.",
                _Reasons.BACKEND_MISSING_INTERFACE
            )

        self._algorithm = algorithm

        self._backend = backend

        max_length = 255 * (algorithm.digest_size // 8)

        if length > max_length:
            raise ValueError(
                "Can not derive keys larger than {0} octets.".format(
                    max_length
                ))

        self._length = length

        if not (info is None or isinstance(info, bytes)):
            raise TypeError("info must be bytes.")

        if info is None:
            info = b""

        self._info = info

        self._used = False
dataset/ETHPy150Open cloudera/hue/desktop/core/ext-py/cryptography-1.3.1/src/cryptography/hazmat/primitives/kdf/hkdf.py/HKDFExpand.__init__/original