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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sign(self, subject, **prefs):
 """ 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`` """

        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)