def _xforwardedforTest(self, header):
        """
        Assert that a request with the given value in its I{X-Forwarded-For}
        header is logged by L{proxiedLogFormatter} the same way it would have
        been logged by L{combinedLogFormatter} but with 172.16.1.2 as the
        client address instead of the normal value.

        @param header: An I{X-Forwarded-For} header with left-most address of
            172.16.1.2.
        """
        reactor = Clock()
        reactor.advance(1234567890)

        timestamp = http.datetimeToLogString(reactor.seconds())
        request = DummyRequestForLogTest(http.HTTPFactory(reactor=reactor))
        expected = http.combinedLogFormatter(timestamp, request).replace(
            u"1.2.3.4", u"172.16.1.2")
        request.requestHeaders.setRawHeaders(b"x-forwarded-for", [header])
        line = http.proxiedLogFormatter(timestamp, request)

        self.assertEqual(expected, line)

        Assert that a request with the given value in its I{X-Forwarded-For}
        header is logged by L{proxiedLogFormatter} the same way it would have
        been logged by L{combinedLogFormatter} but with 172.16.1.2 as the
        client address instead of the normal value.

        @param header: An I{X-Forwarded-For} header with left-most address of
            172.16.1.2.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_web.py