def test_encodeWithCompression(self):
        """
        If a compression dictionary is passed to it, L{Name.encode} uses offset
        information from it to encode its name with references to existing
        labels in the stream instead of including another copy of them in the
        output.  It also updates the compression dictionary with the location of
        the name it writes to the stream.
        """
        name = dns.Name(b"foo.example.com")
        compression = {b"example.com": 0x17}

        # Some bytes already encoded into the stream for this message
        previous = b"some prefix to change .tell()"
        stream = BytesIO()
        stream.write(previous)

        # The position at which the encoded form of this new name will appear in
        # the stream.
        expected = len(previous) + dns.Message.headerSize
        name.encode(stream, compression)
        self.assertEqual(
            b"\x03foo\xc0\x17",
            stream.getvalue()[len(previous):])
        self.assertEqual(
            {b"example.com": 0x17, b"foo.example.com": expected},
            compression)

        If a compression dictionary is passed to it, L{Name.encode} uses offset
        information from it to encode its name with references to existing
        labels in the stream instead of including another copy of them in the
        output.  It also updates the compression dictionary with the location of
        the name it writes to the stream.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/names/test/test_dns.py