def sign(self, pkey, digest):
        """
        Sign the certificate signing request with this key and digest type.

        :param pkey: The key pair to sign with.
        :type pkey: :py:class:`PKey`
        :param digest: The name of the message digest to use for the signature,
            e.g. :py:data:`b"sha256"`.
        :type digest: :py:class:`bytes`
        :return: ``None``
        """
        if pkey._only_public:
            raise ValueError("Key has only public part")

        if not pkey._initialized:
            raise ValueError("Key is uninitialized")

        digest_obj = _lib.EVP_get_digestbyname(_byte_string(digest))
        if digest_obj == _ffi.NULL:
            raise ValueError("No such digest method")

        sign_result = _lib.X509_REQ_sign(self._req, pkey._pkey, digest_obj)
        _openssl_assert(sign_result > 0)

        Sign the certificate signing request with this key and digest type.

        :param pkey: The key pair to sign with.
        :type pkey: :py:class:`PKey`
        :param digest: The name of the message digest to use for the signature,
            e.g. :py:data:`b"sha256"`.
        :type digest: :py:class:`bytes`
        :return: ``None``
        
stackoverflow/venv/lib/python3.6/site-packages/OpenSSL/crypto.py