def setHost(self, host, port, ssl=0):
        """
        Change the host and port the request thinks it's using.

        @type host: C{bytes}
        @param host: The value to which to change the host header.

        @type ssl: C{bool}
        @param ssl: A flag which, if C{True}, indicates that the request is
            considered secure (if C{True}, L{isSecure} will return C{True}).
        """
        self._forceSSL = ssl # set first so isSecure will work
        if self.isSecure():
            default = 443
        else:
            default = 80
        if port == default:
            hostHeader = host
        else:
            hostHeader = host + b":" + intToBytes(port)
        self.requestHeaders.addRawHeader(b"host", hostHeader)

        Change the host and port the request thinks it's using.

        @type host: C{bytes}
        @param host: The value to which to change the host header.

        @type ssl: C{bool}
        @param ssl: A flag which, if C{True}, indicates that the request is
            considered secure (if C{True}, L{isSecure} will return C{True}).
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/requesthelper.py