code: def sign(self, subject, **prefs):
          
          sig_type = SignatureType.BinaryDocument
          hash_algo = prefs.pop('hash', None)
  
          if subject is None:
              sig_type = SignatureType.Timestamp
  
          if isinstance(subject, PGPMessage):
              if subject.type == 'cleartext':
                  sig_type = SignatureType.CanonicalDocument
  
              subject = subject.message
  
          sig = PGPSignature.new(sig_type, self.key_algorithm, hash_algo, self.fingerprint.keyid)
  
          return self._sign(subject, sig, **prefs)
docs: Sign text, a message, or a timestamp using this key.
  
          :param subject: The text to be signed
          :type subject: ``str``, :py:obj:`~pgpy.PGPMessage`, ``None``
          :raises: :py:exc:`~pgpy.errors.PGPError` if the key is passphrase-protected and has not been unlocked
          :raises: :py:exc:`~pgpy.errors.PGPError` if the key is public
          :returns: :py:obj:`PGPSignature`
  
          The following optional keyword arguments can be used with :py:meth:`PGPKey.sign`, as well as
          :py:meth:`PGPKey.certify`,  :py:meth:`PGPKey.revoke`, and :py:meth:`PGPKey.bind`:
  
          :keyword expires: Set an expiration date for this signature
          :type expires: :py:obj:`~datetime.datetime`, :py:obj:`~datetime.timedelta`
          :keyword notation: Add arbitrary notation data to this signature.
          :type notation: ``dict``
          :keyword policy_uri: Add a URI to the signature that should describe the policy under which the signature
                               was issued.
          :type policy_uri: ``str``
          :keyword revocable: If ``False``, this signature will be marked non-revocable
          :type revocable: ``bool``
          :keyword user: Specify which User ID to use when creating this signature. Also adds a "Signer's User ID"
                         to the signature.
          :type user: ``str``