Basic relay proxy in Python

proxy.py

#!/usr/bin/env python3
import threading
import socket

class Relay(threading.Thread):
  def __init__(self, stop_listening, socket_a, socket_b, direction_indicator):
    super().__init__()
    self._stop_listening = stop_listening
    self._socket_a = socket_a
    self._socket_b = socket_b
    self._direction_indicator = direction_indicator
  
  def parse_chunk(self, chunk):
    return chunk

  def run(self):
    while not self._stop_listening.is_set():
      try:
        data = self._socket_a.recv(1024)
        if not data:
          break

        print(self._direction_indicator, self.parse_chunk(data))
        self._socket_b.send(data)
      except RuntimeError as error:
        print(error)
        break

class TcpProxy:
  def __init__(self, port, destination, relay_class=Relay):
    self._destination = destination
    self._relay_class = relay_class

    self._socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    self._socket.bind(("0.0.0.0", port))

  def listen(self, backlog=100):
    self._socket.listen(backlog)
    stop_event = threading.Event()

    while not stop_event.is_set():
      try:
        client, _ = self._socket.accept()
        print("client connected")

        destination_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        destination_socket.connect(self._destination)

        relay_a = self._relay_class(stop_event, client, destination_socket, "in >>\t")
        relay_a.start()

        relay_b = self._relay_class(stop_event, destination_socket, client, "<< out\t")
        relay_b.start()
      except KeyboardInterrupt:
        stop_event.set()

if __name__ == "__main__":
  proxy = TcpProxy(9000, ("localhost", 9001))
  proxy.listen()