def test_earlyHeaders(self):
        """
        When a connection is made, L{HTTPPagerGetter} sends the headers from
        its factory's C{headers} dict.  If I{Host} or I{Content-Length} is
        present in this dict, the values are not sent, since they are sent with
        special values before the C{headers} dict is processed.  If
        I{User-Agent} is present in the dict, it overrides the value of the
        C{agent} attribute of the factory.  If I{Cookie} is present in the
        dict, its value is added to the values from the factory's C{cookies}
        attribute.
        """
        factory = client.HTTPClientFactory(
            b'http://foo/bar',
            agent=b"foobar",
            cookies={b'baz': b'quux'},
            postdata=b"some data",
            headers={
                b'Host': b'example.net',
                b'User-Agent': b'fooble',
                b'Cookie': b'blah blah',
                b'Content-Length': b'12981',
                b'Useful': b'value'})
        transport = StringTransport()
        protocol = client.HTTPPageGetter()
        protocol.factory = factory
        protocol.makeConnection(transport)
        result = transport.value()
        for expectedHeader in [
            b"Host: example.net\r\n",
            b"User-Agent: foobar\r\n",
            b"Content-Length: 9\r\n",
            b"Useful: value\r\n",
            b"connection: close\r\n",
            b"Cookie: blah blah; baz=quux\r\n"]:
            self.assertIn(expectedHeader, result)

        When a connection is made, L{HTTPPagerGetter} sends the headers from
        its factory's C{headers} dict.  If I{Host} or I{Content-Length} is
        present in this dict, the values are not sent, since they are sent with
        special values before the C{headers} dict is processed.  If
        I{User-Agent} is present in the dict, it overrides the value of the
        C{agent} attribute of the factory.  If I{Cookie} is present in the
        dict, its value is added to the values from the factory's C{cookies}
        attribute.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_webclient.py