Twistd echo and shout example of a telnet service with a background task.

telnet_echo.py

"""
telnet_echo.py by @TimelessP
https://gist.github.com/TimelessP/5abce3c64c4b671d79121d1e459aecb3
Run using:
`twistd -ny telnet_echo.py`
"""
from twisted.application.internet import TCPServer
from twisted.application.service import Application
from twisted.conch.telnet import TelnetProtocol, TelnetTransport
from twisted.internet.protocol import ServerFactory
from twisted.internet import task
from twisted.python.failure import Failure
from twisted.logger import Logger


class TelnetEcho(TelnetProtocol):
    log = Logger()

    # When the connection disconnects, remove it from the list of clients.
    def __init__(self):
        self.client = None

    def connectionLost(self, reason: Failure):
        global state
        for client in state["clients"]:
            if client["client"] == self:
                state["clients"].remove(client)
                break

        self.log.info(f"Connection lost: {self.client['client'].transport.getPeer()}, {reason}")

    def connectionMade(self):
        global state
        self.log.info(f"Connection made: {self.transport.getPeer()}")
        self.client = {"client": self, "stdin": [], "stdout": []}
        state["clients"].append(self.client)

    def dataReceived(self, data):
        global state
        self.client["stdin"].append(data)

        message = f"{state['count']}: {repr(data.decode('utf-8'))}, clients: {len(state['clients'])}\r\n"
        message_bytes = message.encode('utf-8')
        self.client["stdout"].append(message_bytes)

        self.client["client"].transport.write(b"\r\n".join(self.client["stdout"]))

        self.client["stdout"] = []


log = Logger()

factory = ServerFactory()
factory.protocol = lambda: TelnetTransport(TelnetEcho)
service = TCPServer(6023, factory)

application = Application("Telnet Echo Server")
service.setServiceParent(application)

state = {
    "count": 0,
    "clients": []
}


def run_every_second():
    state["count"] += 1
    for client in state["clients"]:
        for inp in client["stdin"]:
            if inp.strip() == b"exit":
                client["client"].transport.loseConnection()
            elif inp.strip().startswith(b"shout "):
                log.info(f"Shouting: src={client['client'].transport.getPeer()}: {inp.strip()[6:]}")
                for each_client in [c for c in state["clients"] if c != client]:
                    # Buffered send that awaits that client's next input before sending.
                    # each_client["stdout"].append(inp[6:])

                    # Unbuffered send.
                    each_client["client"].transport.write(inp[6:])

        client["stdin"] = []


# Set up a looping call task that runs once every second.
looping = task.LoopingCall(run_every_second)
looping.start(1.0)