import http.server, socket, threading

class Handler(http.server.BaseHTTPRequestHandler):
    protocol_version = "HTTP/1.1"
    def do_GET(self):
        body = ("Host: %r\n" % self.headers.get_all("Host")).encode()
        self.send_response(200)
        self.send_header("Content-Length", str(len(body)))
        self.end_headers()
        self.wfile.write(body)
    def log_message(self, *args): pass

server = http.server.ThreadingHTTPServer(("127.0.0.1", 0), Handler)
threading.Thread(target=server.serve_forever, daemon=True).start()

for request in [b"GET / HTTP/1.1\r\n\r\n",
                b"GET / HTTP/1.1\r\nHost: a.example\r\nHost: b.example\r\n\r\n",
                b"GET / HTTP/1.1\r\nHost: a b\r\n\r\n"]:
    with socket.create_connection(server.server_address, timeout=2) as s:
        s.sendall(request)
        s.shutdown(socket.SHUT_WR)
        lines = s.makefile("rb").read().split(b"\r\n")
    print(request, "->", lines[0], lines[-1])
server.shutdown()

b'GET / HTTP/1.1\r\n\r\n' -> b'HTTP/1.1 200 OK' b'Host: None\n'
b'GET / HTTP/1.1\r\nHost: a.example\r\nHost: b.example\r\n\r\n' -> b'HTTP/1.1 200 OK' b"Host: ['a.example', 'b.example']\n"
b'GET / HTTP/1.1\r\nHost: a b\r\n\r\n' -> b'HTTP/1.1 200 OK' b"Host: ['a b']\n"