code: def from_seed(cls, seed, encoder=encoding.RawEncoder):
          
          # decode the seed
          seed = encoder.decode(seed)
          # Verify the given seed type and size are correct
          if not (isinstance(seed, bytes) and len(seed) == cls.SEED_SIZE):
              raise exc.TypeError(("PrivateKey seed must be a {0} bytes long "
                                   "binary sequence").format(cls.SEED_SIZE)
                                  )
          # generate a raw keypair from the given seed
          raw_pk, raw_sk = nacl.bindings.crypto_box_seed_keypair(seed)
          # construct a instance from the raw secret key
          return cls(raw_sk)
docs: Generate a PrivateKey using a deterministic construction
          starting from a caller-provided seed
  
          .. warning:: The seed **must** be high-entropy; therefore,
              its generator **must** be a cryptographic quality
              random function like, for example, :func:`~nacl.utils.random`.
  
          .. warning:: The seed **must** be protected and remain secret.
              Anyone who knows the seed is really in possession of
              the corresponding PrivateKey.
  
          :param seed: The seed used to generate the private key
          :rtype: :class:`~nacl.public.PrivateKey`