def simpleVerifyHostname(connection, hostname):
    """
    Check only the common name in the certificate presented by the peer and
    only for an exact match.

    This is to provide I{something} in the way of hostname verification to
    users who haven't installed C{service_identity}. This check is overly
    strict, relies on a deprecated TLS feature (you're supposed to ignore the
    commonName if the subjectAlternativeName extensions are present, I
    believe), and lots of valid certificates will fail.

    @param connection: the OpenSSL connection to verify.
    @type connection: L{OpenSSL.SSL.Connection}

    @param hostname: The hostname expected by the user.
    @type hostname: L{unicode}

    @raise twisted.internet.ssl.VerificationError: if the common name and
        hostname don't match.
    """
    commonName = connection.get_peer_certificate().get_subject().commonName
    if commonName != hostname:
        raise SimpleVerificationError(repr(commonName) + "!=" +
                                      repr(hostname))

    Check only the common name in the certificate presented by the peer and
    only for an exact match.

    This is to provide I{something} in the way of hostname verification to
    users who haven't installed C{service_identity}. This check is overly
    strict, relies on a deprecated TLS feature (you're supposed to ignore the
    commonName if the subjectAlternativeName extensions are present, I
    believe), and lots of valid certificates will fail.

    @param connection: the OpenSSL connection to verify.
    @type connection: L{OpenSSL.SSL.Connection}

    @param hostname: The hostname expected by the user.
    @type hostname: L{unicode}

    @raise twisted.internet.ssl.VerificationError: if the common name and
        hostname don't match.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/_sslverify.py