def sign(self, private_key, algorithm, backend):
        """
        Signs the request using the requestor's private key.
        """
        if self._subject_name is None:
            raise ValueError("A CertificateSigningRequest must have a subject")
        return backend.create_x509_csr(self, private_key, algorithm)

        Signs the request using the requestor's private key.
        
stackoverflow/venv/lib/python3.6/site-packages/cryptography/x509/base.py